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Classification and external resources
EM of influenza virus.jpg
TEM of negatively stained influenza virions, magnified approximately 100,000 times
ICD-10 J10, J11
ICD-9 487
DiseasesDB 6791
MedlinePlus 000080
eMedicine med/1170 ped/3006
MeSH D007251

Influenza, commonly known as "the flu", is an infectious disease of birds and mammals caused by RNA viruses of the family Orthomyxoviridae, the influenza viruses. The most common symptoms are chills, fever, runny nose, sore throat, muscle pains, headache (often severe), coughing, weakness/fatigue and general discomfort. Although it is often confused with other influenza-like illnesses, especially the common cold, influenza is a more severe disease caused by a different type of virus.[1] Influenza may produce nausea and vomiting, particularly in children,[2] but these symptoms are more common in the unrelated gastroenteritis, which is sometimes inaccurately referred to as "stomach flu" or "24-hour flu".[3]

Typically, influenza is transmitted through the air by coughs or sneezes, creating aerosols containing the virus. Influenza can also be transmitted by direct contact with bird droppings or nasal secretions, or through contact with contaminated surfaces. Airborne aerosols have been thought to cause most infections, although which means of transmission is most important is not absolutely clear.[4] Influenza viruses can be inactivated by sunlight, disinfectants and detergents.[5][6] As the virus can be inactivated by soap, frequent hand washing reduces the risk of infection.[7] Flu can occasionally lead to pneumonia, either direct viral pneumonia or secondary bacterial pneumonia, even for persons who are usually very healthy.[8][9][10] In particular it is a warning sign if a child (or presumably an adult) seems to be getting better and then relapses with a high fever as this relapse may be bacterial pneumonia.[11] Another warning sign is if the person starts to have trouble breathing.[10]

Vaccinations against influenza are usually made available to people in developed countries.[12] Farmed poultry is often vaccinated to avoid decimation of the flocks.[13] The most common human vaccine is the trivalent influenza vaccine (TIV) that contains purified and inactivated antigens from three viral strains. Typically, this vaccine includes material from two influenza A virus subtypes and one influenza B virus strain.[14] The TIV carries no risk of transmitting the disease. A vaccine formulated for one year may be ineffective in the following year, since the influenza virus evolves rapidly, and new strains quickly replace the older ones. Antiviral drugs such as the neuraminidase inhibitors oseltamivir among others have been used to treat influenza.[15] Their benefits in those who are otherwise healthy do not appear to be greater than their risks.[16] No benefit has been found in those with other health problems.[16][17]

Influenza spreads around the world in seasonal epidemics, resulting in about three to five million yearly cases of severe illness and about 250,000 to 500,000 yearly deaths,[18] rising to millions in some pandemic years. In the 20th century three influenza pandemics occurred, each caused by the appearance of a new strain of the virus in humans, and killed tens of millions of people. Often, new influenza strains appear when an existing flu virus spreads to humans from another animal species, or when an existing human strain picks up new genes from a virus that usually infects birds or pigs. An avian strain named H5N1 raised the concern of a new influenza pandemic after it emerged in Asia in the 1990s, but it has not evolved to a form that spreads easily between people.[19] In April 2009 a novel flu strain evolved that combined genes from human, pig, and bird flu. Initially dubbed "swine flu" and also known as influenza A/H1N1, it emerged in Mexico, the United States, and several other nations. The World Health Organization officially declared the outbreak to be a pandemic on 11 June 2009 (see 2009 flu pandemic). The WHO's declaration of a pandemic level 6 was an indication of spread, not severity, the strain actually having a lower mortality rate than common flu outbreaks.[20]

Signs and symptoms

Most sensitive symptoms for diagnosing influenza[21]
Symptom: sensitivity specificity
Fever 68–86% 25–73%
Cough 84–98% 7–29%
Nasal congestion 68–91% 19–41%
Symptoms of influenza,[22] with fever and cough the most common symptoms.[21]

Approximately 33% of people with influenza are asymptomatic.[23]

Symptoms of influenza can start quite suddenly one to two days after infection. Usually the first symptoms are chills or a chilly sensation, but fever is also common early in the infection, with body temperatures ranging from 38 to 39 °C (approximately 100 to 103 °F).[24] Many people are so ill that they are confined to bed for several days, with aches and pains throughout their bodies, which are worse in their backs and legs.[2] Symptoms of influenza may include:

It can be difficult to distinguish between the common cold and influenza in the early stages of these infections,[1] but a flu can be identified by a high fever with a sudden onset and extreme fatigue. Influenza is a mixture of symptoms of common cold and pneumonia, body ache, headache, and fatigue. Diarrhea is not normally a symptom of influenza in adults,[21] although it has been seen in some human cases of the H5N1 "bird flu"[29] and can be a symptom in children.[26] The symptoms most reliably seen in influenza are shown in the table to the right.[21]

Since antiviral drugs are effective in treating influenza if given early (see treatment section, below), it can be important to identify cases early. Of the symptoms listed above, the combinations of fever with cough, sore throat and/or nasal congestion can improve diagnostic accuracy.[30] Two decision analysis studies[31][32] suggest that during local outbreaks of influenza, the prevalence will be over 70%,[32] and thus patients with any of these combinations of symptoms may be treated with neuraminidase inhibitors without testing. Even in the absence of a local outbreak, treatment may be justified in the elderly during the influenza season as long as the prevalence is over 15%.[32]


On the more serious side, influenza can occasionally cause either direct viral or secondary bacterial pneumonia.[9][10] The obvious symptom is trouble breathing. In addition, if a child (or presumably an adult) seems to be getting better and then relapses with a high fever, that is a danger sign since this relapse can be bacterial pneumonia.[11]


Types of virus



These viruses are only distantly related to the human parainfluenza viruses, which are RNA viruses belonging to the paramyxovirus family that are a common cause of respiratory infections in children such as croup,[35] but can also cause a disease similar to influenza in adults.[36]

Influenzavirus A

This genus has one species, influenza A virus. Wild aquatic birds are the natural hosts for a large variety of influenza A. Occasionally, viruses are transmitted to other species and may then cause devastating outbreaks in domestic poultry or give rise to human influenza pandemics.[37] The type A viruses are the most virulent human pathogens among the three influenza types and cause the most severe disease. The influenza A virus can be subdivided into different serotypes based on the antibody response to these viruses.[38] The serotypes that have been confirmed in humans, ordered by the number of known human pandemic deaths, are:

Influenzavirus B

Influenza virus nomenclature (for a Fujian flu virus)

This genus has one species, influenza B virus. Influenza B almost exclusively infects humans[38] and is less common than influenza A. The only other animals known to be susceptible to influenza B infection are the seal[40] and the ferret.[41] This type of influenza mutates at a rate 2–3 times slower than type A[42] and consequently is less genetically diverse, with only one influenza B serotype.[38] As a result of this lack of antigenic diversity, a degree of immunity to influenza B is usually acquired at an early age. However, influenza B mutates enough that lasting immunity is not possible.[43] This reduced rate of antigenic change, combined with its limited host range (inhibiting cross species antigenic shift), ensures that pandemics of influenza B do not occur.[44]

Influenzavirus C

This genus has one species, influenza C virus, which infects humans, dogs and pigs, sometimes causing both severe illness and local epidemics.[45][46] However, influenza C is less common than the other types and usually only causes mild disease in children.[47][48]

Structure, properties, and subtype nomenclature

Influenzaviruses A, B and C are very similar in overall structure.[49] The virus particle is 80–120 nanometers in diameter and usually roughly spherical, although filamentous forms can occur.[50][51] These filamentous forms are more common in influenza C, which can form cordlike structures up to 500 micrometers long on the surfaces of infected cells.[52] However, despite these varied shapes, the viral particles of all influenza viruses are similar in composition.[52] These are made of a viral envelope containing two main types of glycoproteins, wrapped around a central core. The central core contains the viral RNA genome and other viral proteins that package and protect this RNA. RNA tends to be single stranded but in special cases it is double.[51] Unusually for a virus, its genome is not a single piece of nucleic acid; instead, it contains seven or eight pieces of segmented negative-sense RNA, each piece of RNA containing either one or two genes, which code for a gene product (protein).[52] For example, the influenza A genome contains 11 genes on eight pieces of RNA, encoding for 11 proteins: hemagglutinin (HA), neuraminidase (NA), nucleoprotein (NP), M1, M2, NS1, NS2(NEP: nuclear export protein), PA, PB1 (polymerase basic 1), PB1-F2 and PB2.[53]

Hemagglutinin (HA) and neuraminidase (NA) are the two large glycoproteins on the outside of the viral particles. HA is a lectin that mediates binding of the virus to target cells and entry of the viral genome into the target cell, while NA is involved in the release of progeny virus from infected cells, by cleaving sugars that bind the mature viral particles.[54] Thus, these proteins are targets for antiviral drugs.[55] Furthermore, they are antigens to which antibodies can be raised. Influenza A viruses are classified into subtypes based on antibody responses to HA and NA. These different types of HA and NA form the basis of the H and N distinctions in, for example, H5N1.[56] There are 16 H and 9 N subtypes known, but only H 1, 2 and 3, and N 1 and 2 are commonly found in humans.[57]



Viruses can replicate only in living cells.[58] Influenza infection and replication is a multi-step process: First, the virus has to bind to and enter the cell, then deliver its genome to a site where it can produce new copies of viral proteins and RNA, assemble these components into new viral particles, and, last, exit the host cell.[52]

Influenza viruses bind through hemagglutinin onto sialic acid sugars on the surfaces of epithelial cells, typically in the nose, throat, and lungs of mammals, and intestines of birds (Stage 1 in infection figure).[59] After the hemagglutinin is cleaved by a protease, the cell imports the virus by endocytosis.[60]

The intracellular details are still being elucidated. It is known that virions converge to the microtubule organizing center, interact with acidic endosomes and finally enter the target endosomes for genome release.[61]

Once inside the cell, the acidic conditions in the endosome cause two events to happen: First, part of the hemagglutinin protein fuses the viral envelope with the vacuole's membrane, then the M2 ion channel allows protons to move through the viral envelope and acidify the core of the virus, which causes the core to dissemble and release the viral RNA and core proteins.[52] The viral RNA (vRNA) molecules, accessory proteins and RNA-dependent RNA polymerase are then released into the cytoplasm (Stage 2).[62] The M2 ion channel is blocked by amantadine drugs, preventing infection.[63]


Negative-sense vRNAs that form the genomes of future viruses, RNA-dependent RNA polymerase, and other viral proteins are assembled into a virion. Hemagglutinin and neuraminidase molecules cluster into a bulge in the cell membrane. The vRNA and viral core proteins leave the nucleus and enter this membrane protrusion (step 6). The mature virus buds off from the cell in a sphere of host phospholipid membrane, acquiring hemagglutinin and neuraminidase with this membrane coat (step 7).[66] As before, the viruses adhere to the cell through hemagglutinin; the mature viruses detach once their neuraminidase has cleaved sialic acid residues from the host cell.[59] After the release of new influenza viruses, the host cell dies.




Influenza virus shedding (the time during which a person might be infectious to another person) begins the day before symptoms appear and virus is then released for five to seven days. People who contract influenza are most infective between the second and third days after infection.[68] The amount of virus shed appears to correlate with fever, with higher amounts of virus shed when temperatures are highest.[69] Children are much more infectious than adults and shed virus from just before they develop symptoms until two weeks after infection.[68][70]

Influenza can be spread in three main ways:[71][72] by direct transmission (when an infected person sneezes mucus directly into the eyes, nose or mouth of another person); the airborne route (when someone inhales the aerosols produced by an infected person coughing, sneezing or spitting) and through hand-to-eye, hand-to-nose, or hand-to-mouth transmission, either from contaminated surfaces or from direct personal contact such as a hand-shake. The relative importance of these three modes of transmission is unclear, and they may all contribute to the spread of the virus.[4][73] In the airborne route, the droplets that are small enough for people to inhale are 0.5 to 5 µm in diameter and inhaling just one droplet might be enough to cause an infection.[71] Although a single sneeze releases up to 40,000 droplets,[74] most of these droplets are quite large and will quickly settle out of the air.[71] How long influenza survives in airborne droplets seems to be influenced by the levels of humidity and UV radiation, with low humidity and a lack of sunlight in winter aiding its survival.[71]

As the influenza virus can persist outside of the body, it can also be transmitted by contaminated surfaces such as banknotes,[75] doorknobs, light switches and other household items.[2] The length of time the virus will persist on a surface varies, with the virus surviving for one to two days on hard, non-porous surfaces such as plastic or metal, for about fifteen minutes from dry paper tissues, and only five minutes on skin.[76] However, if the virus is present in mucus, this can protect it for longer periods (up to 17 days on banknotes).[71][75] Avian influenza viruses can survive indefinitely when frozen.[77] They are inactivated by heating to 56 °C (133 °F) for a minimum of 60 minutes, as well as by acids (at pH <2).[77]



The mechanisms by which influenza infection causes symptoms in humans have been studied intensively. One of the mechanisms is believed to be the inhibition of adrenocorticotropic hormone (ACTH) resulting in lowered cortisol levels.[78] Knowing which genes are carried by a particular strain can help predict how well it will infect humans and how severe this infection will be (that is, predict the strain's pathophysiology).[46][79]


The viral hemagglutinin protein is responsible for determining both which species a strain can infect and where in the human respiratory tract a strain of influenza will bind.[80] Strains that are easily transmitted between people have hemagglutinin proteins that bind to receptors in the upper part of the respiratory tract, such as in the nose, throat and mouth. In contrast, the highly lethal H5N1 strain binds to receptors that are mostly found deep in the lungs.[81] This difference in the site of infection may be part of the reason why the H5N1 strain causes severe viral pneumonia in the lungs, but is not easily transmitted by people coughing and sneezing.[82][83]

Common symptoms of the flu such as fever, headaches, and fatigue are the result of the huge amounts of proinflammatory cytokines and chemokines (such as interferon or tumor necrosis factor) produced from influenza-infected cells.[1][84] In contrast to the rhinovirus that causes the common cold, influenza does cause tissue damage, so symptoms are not entirely due to the inflammatory response.[85] This massive immune response might produce a life-threatening cytokine storm. This effect has been proposed to be the cause of the unusual lethality of both the H5N1 avian influenza,[86] and the 1918 pandemic strain.[87][88] However, another possibility is that these large amounts of cytokines are just a result of the massive levels of viral replication produced by these strains, and the immune response does not itself contribute to the disease.[89]



Giving an influenza vaccination

The influenza vaccine is recommended by the World Health Organization and United States Centers for Disease Control and Prevention for high-risk groups, such as children, the elderly, health care workers, and people who have chronic illnesses such as asthma, diabetes, heart disease, or are immuno-compromised among others.[90][91] In healthy adults it is modestly effective in decreasing the amount of influenza-like symptoms in a population.[92] Evidence is supportive of a decreased rate of influenza in children over the age of two.[93] In those with chronic obstructive pulmonary disease vaccination reduces exacerbations,[94] it is not clear if it reduces asthma exacerbations.[95] Evidence supports a lower rate of influenza-like illness in many groups who are immunocompromised such as those with: HIV/AIDS, cancer, and post organ transplant.[96] In those at high risk immunization may reduce the risk of heart disease.[97] Whether immunizing health care workers effects patient outcomes is controversial with some reviews finding insufficient evidence[98][99] and others finding tentative evidence.[100][101]

Due to the high mutation rate of the virus, a particular influenza vaccine usually confers protection for no more than a few years. Every year, the World Health Organization predicts which strains of the virus are most likely to be circulating in the next year (see Historical annual reformulations of the influenza vaccine), allowing pharmaceutical companies to develop vaccines that will provide the best immunity against these strains.[102] The vaccine is reformulated each season for a few specific flu strains but does not include all the strains active in the world during that season. It takes about six months for the manufacturers to formulate and produce the millions of doses required to deal with the seasonal epidemics; occasionally, a new or overlooked strain becomes prominent during that time.[103] It is also possible to get infected just before vaccination and get sick with the strain that the vaccine is supposed to prevent, as the vaccine takes about two weeks to become effective.[104]


The cost-effectiveness of seasonal influenza vaccination has been widely evaluated for different groups and in different settings.[106] It has generally been found to be a cost-effective intervention, especially in children[107] and the elderly,[108] however the results of economic evaluations of influenza vaccination have often been found to be dependent on key assumptions.[109][110]

Infection control

Reasonably effective ways to reduce the transmission of influenza include good personal health and hygiene habits such as: not touching your eyes, nose or mouth;[111] frequent hand washing (with soap and water, or with alcohol-based hand rubs);[112] covering coughs and sneezes; avoiding close contact with sick people; and staying home yourself if you are sick. Avoiding spitting is also recommended.[113] Although face masks might help prevent transmission when caring for the sick,[114][115] there is mixed evidence on beneficial effects in the community.[113][116] Smoking raises the risk of contracting influenza, as well as producing more severe disease symptoms.[117][118]

Since influenza spreads through both aerosols and contact with contaminated surfaces, surface sanitizing may help prevent some infections.[119] Alcohol is an effective sanitizer against influenza viruses, while quaternary ammonium compounds can be used with alcohol so that the sanitizing effect lasts for longer.[120] In hospitals, quaternary ammonium compounds and bleach are used to sanitize rooms or equipment that have been occupied by patients with influenza symptoms.[120] At home, this can be done effectively with a diluted chlorine bleach.[121]

During past pandemics, closing schools, churches and theaters slowed the spread of the virus but did not have a large effect on the overall death rate.[122][123] It is uncertain if reducing public gatherings, by for example closing schools and workplaces, will reduce transmission since people with influenza may just be moved from one area to another; such measures would also be difficult to enforce and might be unpopular.[113] When small numbers of people are infected, isolating the sick might reduce the risk of transmission.[113]


People with the flu are advised to get plenty of rest, drink plenty of liquids, avoid using alcohol and tobacco and, if necessary, take medications such as acetaminophen (paracetamol) to relieve the fever and muscle aches associated with the flu.[124] Children and teenagers with flu symptoms (particularly fever) should avoid taking aspirin during an influenza infection (especially influenza type B), because doing so can lead to Reye's syndrome, a rare but potentially fatal disease of the liver.[125] Since influenza is caused by a virus, antibiotics have no effect on the infection; unless prescribed for secondary infections such as bacterial pneumonia. Antiviral medication may be effective, but some strains of influenza can show resistance to the standard antiviral drugs and there is concern about the quality of the research.[126]


The two classes of antiviral drugs used against influenza are neuraminidase inhibitors (oseltamivir and zanamivir) and M2 protein inhibitors (adamantane derivatives).

Neuraminidase inhibitors

Overall the benefits of neuraminidase inhibitors in those who are otherwise healthy do not appear to be greater than the risks.[16] There does not appear to be any benefit in those with other health problems.[16] In those believed to have the flu they decreased the length of time symptoms are present by slightly less than a day but do not appear to affect the risk of complications such as needing hospitalization or pneumonia.[17] Previous to 2013 the benefits were unclear as the manufacturer (Roche) refused to release trial data for independent analysis.[127]

M2 inhibitors

The antiviral drugs amantadine and rimantadine block a viral ion channel (M2 protein) and prevent the virus from infecting cells.[63] These drugs are sometimes effective against influenza A if given early in the infection but are ineffective against influenza B viruses, which lack the M2 drug target.[128] Measured resistance to amantadine and rimantadine in American isolates of H3N2 has increased to 91% in 2005.[129] This high level of resistance may be due to the easy availability of amantadines as part of over-the-counter cold remedies in countries such as China and Russia,[130] and their use to prevent outbreaks of influenza in farmed poultry.[131][132] The CDC recommended against using M2 inhibitors during the 2005–06 influenza season due to high levels of drug resistance.[133]


Influenza's effects are much more severe and last longer than those of the common cold. Most people will recover completely in about one to two weeks, but others will develop life-threatening complications (such as pneumonia). Thus, influenza can be deadly, especially for the weak, young and old, or chronically ill.[56] People with a weak immune system, such as people with advanced HIV infection or transplant patients (whose immune systems are medically suppressed to prevent transplant organ rejection), suffer from particularly severe disease.[134] Pregnant women and young children are also at a high risk for complications.[135]

The flu can worsen chronic health problems. People with emphysema, chronic bronchitis or asthma may experience shortness of breath while they have the flu, and influenza may cause worsening of coronary heart disease or congestive heart failure.[136] Smoking is another risk factor associated with more serious disease and increased mortality from influenza.[137]


In some cases, an autoimmune response to an influenza infection may contribute to the development of Guillain-Barré syndrome.[139] However, as many other infections can increase the risk of this disease, influenza may only be an important cause during epidemics.[139][140] This syndrome has been believed to also be a rare side effect of influenza vaccines. One review gives an incidence of about one case per million vaccinations.[141] Getting infected by influenza itself increases both the risk of death (up to 1 in 10,000) and increases the risk of developing GBS to a much higher level than the highest level of suspected vaccine involvement (approx. 10 times higher by recent estimates).[142][143]


Seasonal variations

Seasonal risk areas for influenza: November–April (blue), April–November (red), and year-round (yellow).


A long-standing puzzle has been why outbreaks of the flu occur seasonally rather than uniformly throughout the year. One possible explanation is that, because people are indoors more often during the winter, they are in close contact more often, and this promotes transmission from person to person. Increased travel due to the Northern Hemisphere winter holiday season may also play a role.[144] Another factor is that cold temperatures lead to drier air, which may dehydrate mucus, preventing the body from effectively expelling virus particles. The virus also survives longer on surfaces at colder temperatures and aerosol transmission of the virus is highest in cold environments (less than 5 °C) with low relative humidity.[145] Indeed, the lower air humidity in winter seems to be the main cause of seasonal influenza transmission in temperate regions.[146][147]

However, seasonal changes in infection rates also occur in tropical regions, and in some countries these peaks of infection are seen mainly during the rainy season.[148] Seasonal changes in contact rates from school terms, which are a major factor in other childhood diseases such as measles and pertussis, may also play a role in the flu. A combination of these small seasonal effects may be amplified by dynamical resonance with the endogenous disease cycles.[149] H5N1 exhibits seasonality in both humans and birds.[150]

An alternative hypothesis to explain seasonality in influenza infections is an effect of vitamin D levels on immunity to the virus.[151] This idea was first proposed by Robert Edgar Hope-Simpson in 1965.[152] He proposed that the cause of influenza epidemics during winter may be connected to seasonal fluctuations of vitamin D, which is produced in the skin under the influence of solar (or artificial) UV radiation. This could explain why influenza occurs mostly in winter and during the tropical rainy season, when people stay indoors, away from the sun, and their vitamin D levels fall.

Epidemic and pandemic spread


As influenza is caused by a variety of species and strains of viruses, in any given year some strains can die out while others create epidemics, while yet another strain can cause a pandemic. Typically, in a year's normal two flu seasons (one per hemisphere), there are between three and five million cases of severe illness and around 500,000 deaths worldwide,[153] which by some definitions is a yearly influenza epidemic.[154] Although the incidence of influenza can vary widely between years, approximately 36,000 deaths and more than 200,000 hospitalizations are directly associated with influenza every year in the United States.[155][156] On average 41,400 people died each year in the United States between 1979 and 2001 from influenza.[157] In 2010 the Centers for Disease Control and Prevention (CDC) in the United States changed the way it reports the 30 year estimates for deaths. Now they are reported as a range from a low of about 3,300 deaths to a high of 49,000 per year.[158]

Roughly three times per century, a pandemic occurs, which infects a large proportion of the world's population and can kill tens of millions of people (see pandemics section). One study estimated that if a strain with similar virulence to the 1918 influenza emerged today, it could kill between 50 and 80 million people.[159]


New influenza viruses are constantly evolving by mutation or by reassortment.[38] Mutations can cause small changes in the hemagglutinin and neuraminidase antigens on the surface of the virus. This is called antigenic drift, which slowly creates an increasing variety of strains until one evolves that can infect people who are immune to the pre-existing strains. This new variant then replaces the older strains as it rapidly sweeps through the human population, often causing an epidemic.[160] However, since the strains produced by drift will still be reasonably similar to the older strains, some people will still be immune to them. In contrast, when influenza viruses reassort, they acquire completely new antigens—for example by reassortment between avian strains and human strains; this is called antigenic shift. If a human influenza virus is produced that has entirely new antigens, everybody will be susceptible, and the novel influenza will spread uncontrollably, causing a pandemic.[161] In contrast to this model of pandemics based on antigenic drift and shift, an alternative approach has been proposed where the periodic pandemics are produced by interactions of a fixed set of viral strains with a human population with a constantly changing set of immunities to different viral strains.[162]

The generation time for influenza (the time from one infection to the next) is very short (only 2 days). This explains why influenza epidemics start and finish in a short time scale of only a few months.[163]

From a public health point of view, flu epidemics spread rapidly and are very difficult to control. Most influenza virus strains are not very infectious and each infected individual will only go on to infect one or two other individuals (the basic reproduction number for influenza is generally around 1.4). However, the generation time for influenza is extremely short: the time from a person becoming infected to when he infects the next person is only two days. The short generation time means that influenza epidemics generally peak at around 2 months and burn out after 3 months: the decision to intervene in an influenza epidemic therefore has to be taken early, and the decision is therefore often made on the back of incomplete data. Another problem is that individuals become infectious before they become symptomatic, which means that putting people in quarantine after they become ill is not an effective public health intervention.[163] For the average person, viral shedding tends to peak on day two whereas symptoms peak on day three.[23]



The word Influenza comes from the Italian language meaning "influence" and refers to the cause of the disease; initially, this ascribed illness to unfavorable astrological influences.[164] Changes in medical thought led to its modification to influenza del freddo, meaning "influence of the cold". The word influenza was first used in English to refer to the disease we know today in 1703 by J. Hugger of the University of Edinburgh in his thesis De Catarrho epidemio, vel Influenza, prout in India occidentali sese ostendit.[165] Archaic terms for influenza include epidemic catarrh, grippe (from the French, first used by Molyneaux in 1694 [166]), sweating sickness, and Spanish fever (particularly for the 1918 flu pandemic strain).[167]


Thermal imaging camera and screen, photographed in an airport terminal in Greece during the 2009 flu pandemic. Thermal imaging can detect elevated body temperature, one of the signs of swine flu.

The symptoms of human influenza were clearly described by Hippocrates roughly 2,400 years ago.[169][170] Although the virus seems to have caused epidemics throughout human history, historical data on influenza are difficult to interpret, because the symptoms can be similar to those of other respiratory diseases.[171][172] The disease may have spread from Europe to the Americas as early as the European colonization of the Americas; since almost the entire indigenous population of the Antilles was killed by an epidemic resembling influenza that broke out in 1493, after the arrival of Christopher Columbus.[173][174]


The most famous and lethal outbreak was the 1918 flu pandemic (Spanish flu pandemic) (type A influenza, H1N1 subtype), which lasted from 1918 to 1919. It is not known exactly how many it killed, but estimates range from 50 to 100 million people.[175][176][177] This pandemic has been described as "the greatest medical holocaust in history" and may have killed as many people as the Black Death.[172] This huge death toll was caused by an extremely high infection rate of up to 50% and the extreme severity of the symptoms, suspected to be caused by cytokine storms.[177] Indeed, symptoms in 1918 were so unusual that initially influenza was misdiagnosed as dengue, cholera, or typhoid. One observer wrote, "One of the most striking of the complications was hemorrhage from mucous membranes, especially from the nose, stomach, and intestine. Bleeding from the ears and petechial hemorrhages in the skin also occurred."[176] The majority of deaths were from bacterial pneumonia, a secondary infection caused by influenza, but the virus also killed people directly, causing massive hemorrhages and edema in the lung.[178]



Known flu pandemics[56][172][180]
1889–1890 flu pandemic
(Asiatic or Russian Flu)[181]
1889–1890 1 million 0.15% possibly H3N8
or H2N2
1918 flu pandemic
(Spanish flu)[182]
1918–1920 20 to 100 million 2% H1N1 5
2009 flu pandemic[183] 2009–2010 18,000 0.03% H1N1 NA

The first influenza virus to be isolated was from poultry, when in 1901 the agent causing a disease called "fowl plague" was passed through Chamberland filters, which have pores that are too small for bacteria to pass through.[184] The etiological cause of influenza, the Orthomyxoviridae family of viruses, was first discovered in pigs by Richard Shope in 1931.[185] This discovery was shortly followed by the isolation of the virus from humans by a group headed by Patrick Laidlaw at the Medical Research Council of the United Kingdom in 1933.[186] However, it was not until Wendell Stanley first crystallized tobacco mosaic virus in 1935 that the non-cellular nature of viruses was appreciated.


The first significant step towards preventing influenza was the development in 1944 of a killed-virus vaccine for influenza by Thomas Francis, Jr.. This built on work by Australian Frank Macfarlane Burnet, who showed that the virus lost virulence when it was cultured in fertilized hen's eggs.[188] Application of this observation by Francis allowed his group of researchers at the University of Michigan to develop the first influenza vaccine, with support from the U.S. Army.[189] The Army was deeply involved in this research due to its experience of influenza in World War I, when thousands of troops were killed by the virus in a matter of months.[176] In comparison to vaccines, the development of anti-influenza drugs has been slower, with amantadine being licensed in 1966 and, almost thirty years later, the next class of drugs (the neuraminidase inhibitors) being developed.[57]

Society and culture


Preventative costs are also high. Governments worldwide have spent billions of U.S. dollars preparing and planning for a potential H5N1 avian influenza pandemic, with costs associated with purchasing drugs and vaccines as well as developing disaster drills and strategies for improved border controls.[193] On 1 November 2005, United States President George W. Bush unveiled the National Strategy to Safeguard Against the Danger of Pandemic Influenza[194] backed by a request to Congress for $7.1 billion to begin implementing the plan.[195] Internationally, on 18 January 2006, donor nations pledged US$2 billion to combat bird flu at the two-day International Pledging Conference on Avian and Human Influenza held in China.[196]

In an assessment of the 2009 H1N1 pandemic on selected countries in the Southern Hemisphere, data suggest that all countries experienced some time-limited and/or geographically isolated socio/economic effects and a temporary decrease in tourism most likely due to fear of 2009 H1N1 disease. It is still too early to determine whether the H1N1 pandemic has caused any long-term economic impacts.[197]


Dr. Terrence Tumpey examining a laboratory-grown reconstruction of the 1918 Spanish flu virus in a biosafety level 3 environment.


Research into new vaccines is particularly important, as current vaccines are very slow and expensive to produce and must be reformulated every year. The sequencing of the influenza genome and recombinant DNA technology may accelerate the generation of new vaccine strains by allowing scientists to substitute new antigens into a previously developed vaccine strain.[199] New technologies are also being developed to grow viruses in cell culture, which promises higher yields, less cost, better quality and surge capacity.[200] Research on a universal influenza A vaccine, targeted against the external domain of the transmembrane viral M2 protein (M2e), is being done at the University of Ghent by Walter Fiers, Xavier Saelens and their team[201][202][203] and has now successfully concluded Phase I clinical trials. There has been some research success towards a "universal flu vaccine" that produces antibodies against proteins on the viral coat which mutate less rapidly, and thus a single shot could potentially provide longer-lasting protection.[204][205][206]


Other animals

Influenza infects many animal species, and transfer of viral strains between species can occur. Birds are thought to be the main animal reservoirs of influenza viruses.[208] Sixteen forms of hemagglutinin and nine forms of neuraminidase have been identified. All known subtypes (HxNy) are found in birds, but many subtypes are endemic in humans, dogs, horses, and pigs; populations of camels, ferrets, cats, seals, mink, and whales also show evidence of prior infection or exposure to influenza.[43] Variants of flu virus are sometimes named according to the species the strain is endemic in or adapted to. The main variants named using this convention are: Bird Flu, Human Flu, Swine Flu, Horse Flu and Dog Flu. (Cat flu generally refers to Feline viral rhinotracheitis or Feline calicivirus and not infection from an influenza virus.) In pigs, horses and dogs, influenza symptoms are similar to humans, with cough, fever and loss of appetite.[43] The frequency of animal diseases are not as well-studied as human infection, but an outbreak of influenza in harbor seals caused approximately 500 seal deaths off the New England coast in 1979–1980.[209] On the other hand, outbreaks in pigs are common and do not cause severe mortality.[43] Vaccines have also been developed to protect poultry from avian influenza. These vaccines can be effective against multiple strains and are used either as part of a preventative strategy, or combined with culling in attempts to eradicate outbreaks.[210]

Bird flu

Flu symptoms in birds are variable and can be unspecific.[211] The symptoms following infection with low-pathogenicity avian influenza may be as mild as ruffled feathers, a small reduction in egg production, or weight loss combined with minor respiratory disease.[212] Since these mild symptoms can make diagnosis in the field difficult, tracking the spread of avian influenza requires laboratory testing of samples from infected birds. Some strains such as Asian H9N2 are highly virulent to poultry and may cause more extreme symptoms and significant mortality.[213] In its most highly pathogenic form, influenza in chickens and turkeys produces a sudden appearance of severe symptoms and almost 100% mortality within two days.[214] As the virus spreads rapidly in the crowded conditions seen in the intensive farming of chickens and turkeys, these outbreaks can cause large economic losses to poultry farmers.



In March 2013, the Chinese government reported three cases of H7N9 influenza infections in humans. Two of whom had died and the third was critically ill. Although the strain of the virus is not thought to spread efficiently between humans,[219][220] by mid-April, at least 82 persons had become ill from H7N9, of which 17 had died. These cases include three small family clusters in Shanghai and one cluster between a neighboring girl and boy in Beijing, raising at least the possibility of human-to-human transmission. WHO points out that one cluster did not have two of the cases lab confirmed and further points out, as a matter of baseline information, that some viruses are able to cause limited human-to-human transmission under conditions of close contact but are not transmissible enough to cause large community outbreaks.[221][222]

Swine flu


In pigs swine influenza produces fever, lethargy, sneezing, coughing, difficulty breathing and decreased appetite.[223] In some cases the infection can cause abortion. Although mortality is usually low, the virus can produce weight loss and poor growth, causing economic loss to farmers.[223] Infected pigs can lose up to 12 pounds of body weight over a 3 to 4 week period.[223] Direct transmission of an influenza virus from pigs to humans is occasionally possible (this is called zoonotic swine flu). In all, 50 human cases are known to have occurred since the virus was identified in the mid-20th century, which have resulted in six deaths.[224]

In 2009, a swine-origin H1N1 virus strain commonly referred to as "swine flu" caused the 2009 flu pandemic, but there is no evidence that it is endemic to pigs (i.e. actually a swine flu) or of transmission from pigs to people, instead the virus is spreading from person to person.[225][226] This strain is a reassortment of several strains of H1N1 that are usually found separately, in humans, birds, and pigs.[227]


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Further reading






Treatment and prevention


External links