Smallpox is a contagious disease caused by a virus called Variola. Smallpox was a major health problem worldwide for thousands of years, with outbreaks occurring from time to time. The last natural outbreak of smallpox in the U.S. occurred in 1949 and smallpox was declared eradicated from the globe in 1980 after a successful worldwide vaccination program. No naturally occurring cases have happened since 1979. When the disease was eliminated, routine vaccination against smallpox was stopped, and the general public no longer has immunity to the disease. Because there are some laboratory samples of the virus remaining, it could possibly be used as a bioterrorist weapon. Smallpox research continues and focuses on the development of diagnostic tests, vaccines and drugs to protect people against smallpox in the event it is used an agent of bioterrorism.
Symptoms usually appear 7 to 19 days after exposure to the disease with the average length being 10 to 14 days. The first symptoms include high fever, head and body aches, and sometimes vomiting. Two to four days after the above symptoms start, a rash will appear. Small red spots will show up first in the mouth and on the tongue, then on the head, spreading down the arms and legs to the hands and feet (including the palms of the hands and soles of the feet). The rash will quickly spread over the entire body, usually within 24 hours. By the third day of the rash, the rash becomes raised bumps that fill with a thick fluid and often have a dimple in the center that looks like a belly button. The bumps then become pustules—sharply raised, round and firm to the touch, as if there is a small ball under the skin. Scabs will form on the pustules, which will fall off after about three weeks, leaving pitted scars. Three weeks after the rash appears, most scabs will have fallen off.
A person is contagious from the beginning of the early symptoms until the last smallpox scab falls off. Four weeks after the rash appears, all scabs should have fallen off. Once all scabs have fallen off, the person is no longer contagious.
In the past people caught smallpox by being in close contact with an infected person. Smallpox can also be spread through direct contact with infected body fluids (blood, saliva, vomit) or personal items such as bedding or clothing that have been used by an infected person. Rarely, smallpox has been spread through the air in places such as buses or buildings. However, exposure for at least 3 hours is necessary for infection to occur. Smallpox is not spread by insects or animals.
Smallpox is indicated by a clear cut febrile illness with simultaneous appearance of lesions when the fever broke. The appearance of a rash is characterized by firm, deep seated vesicles or pustules in the same stage of development without other apparent cause. Laboratory confirmation of infection is done using polymerase chain reaction (PCR) identification of variola DNA in a clinical specimen, OR isolation of smallpox (variola) virus from a clinical specimen, confirmed by variola PCR.
There is no medicine that will treat smallpox once the rash appears, but some antiviral drugs may help treat it or prevent it from getting worse. Vaccination within 3 days of exposure will usually prevent or lessen the severity of smallpox symptoms.
There is no lifetime immunity from the vaccine. Past experience indicates that the first dose of the vaccine offers high level protection from smallpox for 3 to 5 years, with decreasing immunity thereafter. If a person is vaccinated again later, immunity lasts longer.
No known human cases worldwide since 1978.
Smallpox can be prevented by the smallpox vaccine. Currently, the smallpox vaccine is not available to the general public because smallpox has been eradicated, and the virus no longer exists in nature. The vaccine should not be given to certain groups of people which include; those that have certain chronic skin conditions (eczema or atopic dermatitis), those with breaks in the skin (poison ivy or acne), those with weakened immune systems, and pregnant women or women trying to become pregnant. For anyone directly exposed to smallpox, the risks associated with smallpox disease are far greater than those posed by the vaccine.
Notification of smallpox is mandatory under the International Health Regulations (IHR). Suspected cases should be immediately reported to local and national health authorities.
Last Reviewed: March 2018