Smallpox is a contagious and often fatal infection caused by the variola virus. It presents in 2 clinical forms: variola major smallpox (historic mortality rate: 30%) and variola minor, which produces a milder smallpox-like illness (historic mortality rate: less than 1%). Smallpox, in all its forms, was declared eradicated in 1980. The virus is stored in only 2 laboratories, the CDC in Atlanta and The Institute for Virus Preparations in Moscow.
Humans are the only known hosts of the variola virus; there are no animal or insect vectors. Smallpox is extremely contagious, with only 5-10 virions sufficient to produce infection. It is easily spread person to person by respiratory droplets and/or contact with bodily fluids, lesions or scabs, and contaminated clothing or bedding. On rare occasions, in enclosed spaces, it has been transmitted by virus carried in the air. Patients are most contagious from about 24 hours before the typical rash first appears until the scabs heal and fall off. The incubation period lasts from 3 to 17 days, during which time the patient is not contagious.
Smallpox plays a significant role in Lake in the Clouds of the Wilderness series — set in 1802. Despite the availability of an effective vaccine, the disease was still common in the late 19th century.
Smallpox was most likely the first disease people tried to prevent by inoculating themselves and was the first disease for which a vaccine was produced. The smallpox vaccine was discovered in 1796 by the British physician Edward Jenner, although at least six people had used the same principles years earlier. Louis Pasteur furthered the concept through his work in microbiology. The immunization was called vaccination because it was derived from a virus affecting cows (Latin:vacca—cow). Smallpox was a contagious and deadly disease, causing the deaths of 20–60% of infected adults and over 80% of infected children. When smallpox was finally eradicated in 1979, it had already killed an estimated 300–500 million people in the 20th century. Wikipedia