when contaminated airborne droplets are inhaled through the nose or mouth. once in the upper airway, the viruses may make their way into the lungs, where they invade the cells lining the airways, alveoli, or lung parenchyma. some viruses such as measles and herpes simplex may reach the lungs via the blood. the invasion of the lungs may lead to varying degrees of cell death. when the immune system responds to the infection, even more lung damage may occur. primarily white blood cells, mainly mononuclear cells, generate the inflammation. as well as damaging the lungs, many viruses simultaneously affect other organs and thus disrupt other body functions. viruses also make the body more susceptible to bacterial infections; in this way, bacterial pneumonia can occur at the same time as viral pneumonia. diagnosis pneumonia is typically diagnosed based on a combination of physical signs and often a chest x-ray. in adults with normal vital signs and a normal lung examination, the diagnosis is unlikely. however, the underlying cause can be difficult to confirm, as there is no definitive test able to distinguish between bacterial and non-bacterial cause. the overall impression of a physician appears to be at least as good as decision rules for making or excluding the diagnosis. diagnosis in children the world health organization has defined pneumonia in children clinically based on either a cough or difficulty breathing and a rapid respiratory rate, chest indrawing, or a decreased level of consciousness. a rapid respiratory rate is defined as greater than 60 breaths per minute in children under 2 months old, greater than 50 breaths per minute in children 2 months to 1 year old, or greater than 40 breaths per minute in children 1 to 5 years old. in children, low oxygen levels and lower chest indrawing are more sensitive than hearing chest crackles with a stethoscope or increased respiratory rate. grunting and nasal flaring may be other useful signs in children less than five years old. lack of wheezing is an indicator of mycoplasma pneumoniae in children with pneumonia, but as an indicator it is not accurate enough to decide whether or not macrolide treatment should be used. the presence of chest pain in children with pneumonia doubles the probability of mycoplasma pneumoniae. diagnosis in adults in general, in adults, investigations are not needed in mild cases. there is a very low risk of pneumonia if all vital signs and auscultation are normal. c-reactive protein (crp) may help support the diagnosis. for those with crp less than 20 mg/l without convincing evidence of pneumonia, antibiotics are not recommended. procalcitonin may help determine the cause and support decisions about who should receive antibiotics. antibiotics are encouraged if the procalcitonin level reaches 0.25 μg/l, strongly encouraged if it reaches 0.5 μg/l, and strongly discouraged if the level is below 0.10 μg/l. in people requiring hospitalization, pulse oximetry, chest radiography and blood tests – including a complete blood count, serum electrolytes, c-reactive protein level, and possibly liver function tests – are recommended. the diagnosis of influenza-like illness can be made based on the signs and symptoms; however, confirmation of an influenza infection requires testing. thus, treatment is frequently based on the presence of influenza in the community or a rapid influenza test. adults 65 years old or older, as well as cigarette smokers and people with ongoing medical conditions are at increased risk for pneumonia. physical exam physical examination may sometimes reveal low blood pressure, high heart rate, or low oxygen saturation. the respiratory rate may be faster than normal, and this may occur a day or two before other signs. examination of the chest may be normal, but it may show decreased expansion on the affected side. harsh breath sounds from the larger airways that are transmitted through the inflamed lung are termed bronchial breathing and are heard on auscultation with a stethoscope. crackles (rales) may be heard over the affected area during inspiration. percussion may be dulled over the affected lung, and increased, rather than decreased, vocal resonance distinguishes pneumonia from a pleural effusion. imaging a chest x-ray showing a very prominent wedge-shaped area of airspace consolidation in the right lung characteristic of acute bacterial lobar pneumonia ct of the chest demonstrating right-sided pneumonia (left side of the image) a chest radiograph is frequently used in diagnosis. in people with mild disease, imaging is needed only in those with potential complications, those not having improved with treatment, or those in which the cause is uncertain. if a person is sufficiently sick to require hospitalization, a chest radiograph is recommended. findings do not always match the severity of disease and do not reliably separate between bacterial and viral infection. x-ray presentations of pneumonia may be classified as lobar pneumonia, bronchopneumonia, lobular pneumonia, and interstitial pneumonia. bacterial, community-acquired pneumonia classically show lung consolidation of one lung segmental lobe, which is known as lobar pneumonia. however, findings may vary, and other patterns are common in other types of pneumonia. aspiration pneumonia may present with bilateral opacities primarily in the bases of the lungs and on the right side. radiographs of viral pneumonia may appear normal, appear hyper-inflated, have bilateral patchy areas, or present similar to bacterial pneumonia with lobar consolidation. radiologic findings may not be present in the early stages of the disease, especially in the presence of dehydration, or may be difficult to interpret in the obese or those with a history of lung disease. complications such as pleural effusion may also be found on chest radiographs. laterolateral chest radiographs can increase the diagnostic accuracy of lung consolidation and pleural effusion. a ct scan can give additional information in indeterminate cases and provide more details in those with an unclear chest radiograph (for example occult pneumonia in chronic obstructive pulmonary disease). they can be used to exclude pulmonary embolism and fungal pneumonia, and detect lung abscesses in those who are not responding to treatments. however, ct scans are more expensive, have a higher dose of radiation, and cannot be done at bedside. lung ultrasound may also be useful in helping to make the diagnosis. ultrasound is radiation free and can be done at bedside. however, ultrasound requires specific skills to operate the machine and interpret the findings. it may be more accurate than chest x-ray. pneumonia seen by ultrasound right middle lobe pneumonia in a child as seen on plain x-ray microbiology in people managed in the community, determining the causative agent is not cost-effective and typically does not alter management. for people who do not respond to treatment, sputum culture should be considered, and culture for mycobacterium tuberculosis should be carried out in persons with a chronic productive cough. microbiological evaluation is also indicated in severe pneumonia, alcoholism, asplenia, immunosuppression, hiv infection, and those being empirically treated for mrsa of pseudomonas. although positive blood culture and pleural fluid culture definitively establish the diagnosis of the type of micro-organism involved, a positive sputum culture has to be interpreted with care for the possibility of colonisation of respiratory tract. testing for other specific organisms may be recommended during outbreaks, for public health reasons. in those hospitalized for severe disease, both sputum and blood cultures are recommended, as well as testing the urine for antigens to legionella and streptococcus. viral infections, can be confirmed via detection of either the virus or its antigens with culture or polymerase chain reaction (pcr), among other techniques. mycoplasma, legionella, streptococcus, and chlamydia can also be detected using pcr techniques on bronchoalveolar lavage and nasopharyngeal swab. the causative agent is determined in only 15% of cases with routine microbiological tests. classification main article: classification of pneumonia pneumonitis refers to lung inflammation; pneumonia refers to pneumonitis, usually due to infection but sometimes non-infectious, that has the additional feature of pulmonary consolidation. pneumonia is most commonly classified by where or how it was acquired: community-acquired, aspiration, healthcare-associated, hospital-acquired, and ventilator-associated pneumonia. it may also be classified by the area of the lung affected: lobar, bronchial pneumonia and acute interstitial pneumonia; or by the causative organism. pneumonia in children may additionally be classified based on signs and symptoms as non-severe, severe, or very severe. the setting in which pneumonia develops is important to treatment, as it correlates to which pathogens are likely suspects, which mechanisms are likely, which antibiotics are likely to work or fail, and which complications can be expected based on the person's health status. community main article: community-acquired pneumonia community-acquired pneumonia (cap) is acquired in the community, outside of health care facilities. compared with healthcare-associated pneumonia, it is less likely to involve multidrug-resistant bacteria. although the latter are no longer rare in cap, they are still less likely. prior stays in healthcare-related environments such as hospitals, nursing homes, or hemodialysis centers or a history of receiving domiciliary care can increase patients’ risk for cap caused by multidrug-resistant bacteria. healthcare health care–associated pneumonia (hcap) is an infection associated with recent exposure to the health care system, including hospitals, outpatient clinics, nursing homes, dialysis centers, chemotherapy treatment, or home care. hcap is sometimes called mcap (medical care–associated pneumonia). people may become infected with pneumonia in a hospital; this is defined as pneumonia not present at the time of admission (symptoms must start at least 48 hours after admission). it is likely to involve hospital-acquired infections, with higher risk of multidrug-resistant pathogens. people in a hospital often have other medical conditions, which may make them more susceptible to pathogens in the hospital. ventilator-associated pneumonia occurs in people breathing with the help of mechanical ventilation. ventilator-associated pneumonia is specifically defined as pneumonia that arises more than 48 to 72 hours after endotracheal intubation. differential diagnosis several diseases can present with similar signs and symptoms to pneumonia, such as: chronic obstructive pulmonary disease, asthma, pulmonary edema, bronchiectasis, lung cancer, and pulmonary emboli. unlike pneumonia, asthma and copd typically present with wheezing, pulmonary edema presents with an abnormal electrocardiogram, cancer and bronchiectasis present with a cough of longer duration, and pulmonary emboli present with acute onset sharp chest pain and shortness of breath. mild pneumonia should be differentiated from upper respiratory tract infection (urti). severe pneumonia should be differentiated from acute heart failure. pulmonary infiltrates that resolved after giving mechanical ventilation should point to heart failure and atelectasis rather than pneumonia. for recurrent pneumonia, underlying lung cancer, metastasis, tuberculosis, a foreign bodies, immunosuppression, and hypersensitivity should be suspected. prevention prevention includes vaccination, environmental measures, and appropriate treatment of other health problems. it is believed that, if appropriate preventive measures were instituted globally, mortality among children could be reduced by 400,000; and, if proper treatment were universally available, childhood deaths could be decreased by another 600,000. vaccination vaccination prevents against certain bacterial and viral pneumonias both in children and adults. influenza vaccines are modestly effective at preventing symptoms of influenza, the centers for disease control and prevention (cdc) recommends yearly influenza vaccination for every person 6 months and older. immunizing health care workers decreases the risk of viral pneumonia among their patients. vaccinations against haemophilus influenzae and streptococcus pneumoniae have good evidence to support their use. there is strong evidence for vaccinating children under the age of 2 against streptococcus pneumoniae (pneumococcal conjugate vaccine). vaccinating children against streptococcus pneumoniae has led to a decreased rate of these infections in adults, because many adults acquire infections from children. a streptococcus pneumoniae vaccine is available for adults, and has been found to decrease the risk of invasive pneumococcal disease by 74%, but there is insufficient evidence to suggest using the pneumococcal vaccine to prevent pneumonia or death in the general adult population. the cdc recommends that young children and adults over the age of 65 receive the pneumococcal vaccine, as well as older children or younger adults who have an increased risk of getting pneumococcal disease. the pneumococcal vaccine has been shown to reduce the risk of community acquired pneumonia in people with chronic obstructive pulmonary disease, but does not reduce mortality or the risk of hospitalization for people with this condition. people with copd are recommended by a number of guidelines to have a pneumococcal vaccination. other vaccines for which there is support for a protective effect against pneumonia include pertussis, varicella, and measles. medications when influenza outbreaks occur, medications such as amantadine or rimantadine may help prevent the condition, but they are associated