Smallpox is transmitted through extended face-to-face contact
, or direct contact with infected body fluids or contaminated objects.
Insects and animals do not transmit smallpox. If aerosolized smallpox were to be released, 90% of the virus would die within 24 hours.
The incubation period averages about 10 to 14 days
(but can range from seven to 19 days) following exposure. Subjects are not contagious at this time.
Initial symptoms include high fever, fatigue, and head and back aches. A characteristic rash that is most prominent on the face, arms and legs follows in two to three days. The rash starts with flat red lesions that evolve at the same rate. Lesions become pus-filled and begin to crust early in the second week. Scabs develop and then separate and fall off after about three to four weeks.
The majority of patients with smallpox recover, but death occurs in up to 30% of cases.
1950s - Worldwide, 15 million cases of smallpox are reported each year in the decade; the highly contagious disease kills more than 500 million people worldwide over the last century.
1977 - The last naturally occurring case of smallpox in the world occurs in Somalia.
1979 - Smallpox meets the criteria for eradication by having no natural cases for two years. There are two sanctioned repositories for stocks of variola, the virus that causes smallpox. They are: the Centers for Disease Control and Prevention in Atlanta, Georgia, and the Russian State Research Center of Virology and Biotechnology in Koltsovo.
1980 - The World Health Organization announces the official eradication of smallpox.
2014 - Six vials containing the smallpox virus are found in an unused storage room at the Food and Drug Administration's Bethesda, Maryland, campus. Later testing shows that at least two of the vials, dating from 1954, contain the live virus.
The oldest known sample of the smallpox-causing variola virus is found within the DNA of a 17th century child mummy
, in a crypt beneath a Lithuanian church, according to a study in the journal Current Biology. The finding shortens the timeline for how long smallpox may have afflicted humans.
In people exposed to smallpox, the vaccine can lessen the severity of or even prevent illness if given within three to four days after exposure.
Vaccine against smallpox contains a live virus called vaccinia, which is related to the variola virus that causes smallpox.
Exposure to the vaccinia pathogen in the vaccine can cause severe complications in rare cases. The people most at risk are those with compromised immune systems, like those with HIV or people undergoing cancer treatment. Also at risk are people with certain skin conditions who may be more sensitive to the virus in the vaccine.
Most Americans under 40 have not been vaccinated. The last smallpox case in the United States was in 1949, and routine vaccination stopped in 1972. Some medical and military personal are still vaccinated.