virus can cross the placenta, but the incidence of congenital smallpox was relatively low. smallpox was not notably infectious in the prodromal period and viral shedding was usually delayed until the appearance of the rash, which was often accompanied by lesions in the mouth and pharynx. the virus can be transmitted throughout the course of the illness, but this happened most frequently during the first week of the rash when most of the skin lesions were intact. infectivity waned in 7 to 10 days when scabs formed over the lesions, but the infected person was contagious until the last smallpox scab fell off. mechanism once inhaled, the variola virus invaded the mucus membranes of the mouth, throat, and respiratory tract. from there, it migrated to regional lymph nodes and began to multiply. in the initial growth phase, the virus seemed to move from cell to cell, but by around the 12th day, widespread lysis of infected cells occurred and the virus could be found in the bloodstream in large numbers, a condition known as viremia. this resulted in the second wave of multiplication in the spleen, bone marrow, and lymph nodes. diagnosis the clinical definition of ordinary smallpox is an illness with acute onset of fever equal to or greater than 38.3 °c (101 °f) followed by a rash characterized by firm, deep-seated vesicles or pustules in the same stage of development without other apparent cause. when a clinical case was observed, smallpox was confirmed using laboratory tests. microscopically, poxviruses produce characteristic cytoplasmic inclusion bodies, the most important of which are known as guarnieri bodies, and are the sites of viral replication. guarnieri bodies are readily identified in skin biopsies stained with hematoxylin and eosin, and appear as pink blobs. they are found in virtually all poxvirus infections but the absence of guarnieri bodies could not be used to rule out smallpox. the diagnosis of an orthopoxvirus infection can also be made rapidly by electron microscopic examination of pustular fluid or scabs. all orthopoxviruses exhibit identical brick-shaped virions by electron microscopy. if particles with the characteristic morphology of herpesviruses are seen this will eliminate smallpox and other orthopoxvirus infections. definitive laboratory identification of variola virus involved growing the virus on chorioallantoic membrane (part of a chicken embryo) and examining the resulting pock lesions under defined temperature conditions. strains were characterized by polymerase chain reaction (pcr) and restriction fragment length polymorphism (rflp) analysis. serologic tests and enzyme linked immunosorbent assays (elisa), which measured variola virus-specific immunoglobulin and antigen were also developed to assist in the diagnosis of infection. chickenpox was commonly confused with smallpox in the immediate post-eradication era. chickenpox and smallpox could be distinguished by several methods. unlike smallpox, chickenpox does not usually affect the palms and soles. additionally, chickenpox pustules are of varying size due to variations in the timing of pustule eruption: smallpox pustules are all very nearly the same size since the viral effect progresses more uniformly. a variety of laboratory methods were available for detecting chickenpox in the evaluation of suspected smallpox cases. smallpox virus lesions on the chorioallantoic membrane of a developing chick. in contrast to the rash in smallpox, the rash in chickenpox occurs mostly on the torso, spreading less to the limbs. an italian female smallpox patient whose skin displayed the characteristics of late-stage confluent maculopapular scarring, 1965. prevention main article: smallpox vaccine components of a modern smallpox vaccination kit including the diluent, a vial of dryvax vaccinia vaccine, and a bifurcated needle. the earliest procedure used to prevent smallpox was inoculation with variola minor virus (a method later known as variolation after the introduction of smallpox vaccine to avoid possible confusion), which likely occurred in india, africa, and china well before the practice arrived in europe. the idea that inoculation originated in india has been challenged, as few of the ancient sanskrit medical texts described the process of inoculation. accounts of inoculation against smallpox in china can be found as early as the late 10th century, and the procedure was widely practiced by the 16th century, during the ming dynasty. if successful, inoculation produced lasting immunity to smallpox. because the person was infected with variola virus, a severe infection could result, and the person could transmit smallpox to others. variolation had a 0.5–2 percent mortality rate, considerably less than the 20–30 percent mortality rate of the disease. two reports on the chinese practice of inoculation were received by the royal society in london in 1700; one by dr. martin lister who received a report by an employee of the east india company stationed in china and another by clopton havers. lady mary wortley montagu observed smallpox inoculation during her stay in the ottoman empire, writing detailed accounts of the practice in her letters, and enthusiastically promoted the procedure in england upon her return in 1718. according to voltaire (1742), the turks derived their use of inoculation from neighbouring circassia. voltaire does not speculate on where the circassians derived their technique from, though he reports that the chinese have practiced it "these hundred years". in 1721, cotton mather and colleagues provoked controversy in boston by inoculating hundreds. after publishing the present method of inoculating for the small-pox in 1767, dr thomas dimsdale was invited to russia to variolate the empress catherine the great of russia and her son, grand duke paul, which he successfully did in 1768. in 1796, edward jenner, a doctor in berkeley, gloucestershire, rural england, discovered that immunity to smallpox could be produced by inoculating a person with material from a cowpox lesion. cowpox is a poxvirus in the same family as variola. jenner called the material used for inoculation vaccine from the root word vacca, which is latin for cow. the procedure was much safer than variolation and did not involve a risk of smallpox transmission. vaccination to prevent smallpox was soon practiced all over the world. during the 19th century, the cowpox virus used for smallpox vaccination was replaced by the vaccinia virus. vaccinia is in the same family as cowpox and variola virus but is genetically distinct from both. the origin of the vaccinia virus and how it came to be in the vaccine are not known. an 1802 cartoon by james gillray of the early controversy surrounding edward jenner's vaccination procedure, showing using his cowpox-derived smallpox vaccine causing cattle to emerge from patients. the current formulation of the smallpox vaccine is a live virus preparation of the infectious vaccinia virus. the vaccine is given using a bifurcated (two-pronged) needle that is dipped into the vaccine solution. the needle is used to prick the skin (usually the upper arm) several times in a few seconds. if successful, a red and itchy bump develops at the vaccine site in three or four days. in the first week, the bump becomes a large blister (called a "jennerian vesicle") which fills with pus and begins to drain. during the second week, the blister begins to dry up, and a scab forms. the scab falls off in the third week, leaving a small scar. the antibodies induced by the vaccinia vaccine are cross-protective for other orthopoxviruses, such as monkeypox, cowpox, and variola (smallpox) viruses. neutralizing antibodies are detectable 10 days after first-time vaccination and seven days after revaccination. historically, the vaccine has been effective in preventing smallpox infection in 95 percent of those vaccinated. smallpox vaccination provides a high level of immunity for three to five years and decreasing immunity thereafter. if a person is vaccinated again later, the immunity lasts even longer. studies of smallpox cases in europe in the 1950s and 1960s demonstrated that the fatality rate among persons vaccinated less than 10 years before exposure was 1.3 percent; it was 7 percent among those vaccinated 11 to 20 years prior, and 11 percent among those vaccinated 20 or more years before infection. by contrast, 52 percent of unvaccinated persons died. a demonstration by medical personnel on use of a bifurcated needle to deliver the smallpox vaccine, 2002. there are side effects and risks associated with the smallpox vaccine. in the past, about 1 out of 1,000 people vaccinated for the first time experienced serious, but non-life-threatening, reactions, including toxic or allergic reaction at the site of the vaccination (erythema multiforme), spread of the vaccinia virus to other parts of the body, and spread to other individuals. potentially life-threatening reactions occurred in 14 to 500 people out of every 1 million people vaccinated for the first time. based on past experience, it is estimated that 1 or 2 people in 1 million (0.000198 percent) who receive the vaccine may die as a result, most often the result of postvaccinial encephalitis or severe necrosis in the area of vaccination (called progressive vaccinia). given these risks, as smallpox became effectively eradicated and the number of naturally occurring cases fell below the number of vaccine-induced illnesses and deaths, routine childhood vaccination was discontinued in the united states in 1972 and was abandoned in most european countries in the early 1970s. routine vaccination of health care workers was discontinued in the u.s. in 1976, and among military recruits in 1990 (although military personnel deploying to the middle east and korea still receive the vaccination). by 1986, routine vaccination had ceased in all countries. it is now primarily recommended for laboratory workers at risk for occupational exposure. however, the possibility of variola virus being used as a biological weapon has rekindled interest in the development of newer vaccines. the smallpox vaccine is also effective in, and therefore administered for, the prevention of monkeypox. treatment smallpox vaccination within three days of exposure will prevent or significantly lessen the severity of smallpox symptoms in the vast majority of people. vaccination four to seven days after exposure can offer some protection from disease or may modify the severity of the disease. other than vaccination, treatment of smallpox is primarily supportive, such as wound care and infection control, fluid therapy, and possible ventilator assistance. flat and hemorrhagic types of smallpox are treated with the same therapies used to treat shock, such as fluid resuscitation. people with semi-confluent and confluent types of smallpox may have therapeutic issues similar to patients with extensive skin burns. in july 2018, the food and drug administration approved tecovirimat, the first drug approved for treatment of smallpox. antiviral treatments have improved since the last large smallpox epidemics, and studies suggest that the antiviral drug cidofovir might be useful as a therapeutic agent. the drug must be administered intravenously, and may cause serious kidney toxicity. acam2000 is a smallpox vaccine developed by acambis. it was approved for use in the united states by the u.s. fda on august 31, 2007. it contains live vaccinia virus, cloned from the same strain used in an earlier vaccine, dryvax. while the dryvax virus was cultured in the skin of calves and freeze-dried, acam2000s virus is cultured in kidney epithelial cells (vero cells) from an african green monkey. efficacy and adverse reaction incidence are similar to dryvax. the vaccine is not routinely available to the us public; it is, however, used in the military and maintained in the strategic national stockpile. in june 2021, brincidofovir was approved for medical use in the united states for the treatment of human smallpox disease caused by variola virus. prognosis smallpox survivor with facial scarring, blindness and white corneal scar in his left eye, 1972 the mortality rate from variola minor is approximately 1%, while the mortality rate from variola major is approximately 30%. ordinary type-confluent is fatal about 50–75% of the time, ordinary-type semi-confluent about 25–50% of the time, in cases where the rash is discrete the case-fatality rate is less than 10%. the overall fatality rate for children younger than 1 year of age is 40–50%. hemorrhagic and flat types have the highest fatality rates. the fatality rate for flat or late hemorrhagic type smallpox is 90% or greater and nearly 100% is observed in cases of early hemorrhagic smallpox. the case-fatality rate for variola minor is 1% or less. there is no evidence of chronic or recurrent infection with variola virus. in cases of flat smallpox in vaccinated people, the condition was extremely rare but less lethal, with one case series showing a 66.7% death rate. in fatal cases of ordinary smallpox, death usually occurs between days 10-16