Smallpox (variola) is a highly contagious, very deadly disease caused by the smallpox virus.

• People can acquire the infection by breathing air exhaled or coughed out by an infected person.
• People have a fever, headache, backache, and rash, sometimes with severe abdominal pain, and they feel very ill.
• The diagnosis is confirmed when the virus is identified in a sample taken from the rash.
• Vaccination within the first few days of exposure can prevent the disease or limit its severity.
• Treatment involves fluids, relief of symptoms, and treatments to maintain blood pressure and help with breathing.

The smallpox virus can exist only in people—not in animals. There are two main forms. The severe form is the most common and is the one of concern. The other form is much less common and much less severe.

Over 200 years ago, a vaccine against smallpox (the first vaccine ever) was developed. The vaccine proved very effective and was given to people throughout the world. The last case of smallpox was reported in 1977. In 1980, the World Health Organization (WHO) declared the disease eliminated and recommended stopping vaccination.

Because the vaccine's protective effects gradually wear off, nearly all people—even those previously vaccinated—are now susceptible to smallpox (see also see Immunization: Smallpox: A Vaccine in the Wings). This lack of protection is a concern only because samples of the virus have been stored, and some people worry that terrorist groups could obtain the virus and release it into the population. The resulting epidemic would be devastating. The virus is stored at two research facilities, one in the United States and one in Russia.

The smallpox virus spreads directly from person to person and is acquired by breathing air contaminated with droplets of moisture breathed or coughed out by an infected person. Contact with clothing or bed linens used by an infected person can also spread the disease. Smallpox usually spreads to people who have close personal contact with an infected person. A large outbreak in a school or workplace would be uncommon. The virus survives no more than 2 days in the environment— less if temperature and humidity are high.

Did You Know... Nearly everyone, even people who were previously vaccinated, is now susceptible to smallpox.

Symptoms and Diagnosis

Symptoms usually begin 7 to 17 days after infection. Infected people develop fever, headache, and backache and feel extremely ill. They may have severe abdominal pain and become delirious. After 2 or 3 days, a rash of flat, red spots develops on the face and arms and inside the mouth, spreading shortly thereafter to the trunk and legs. People are contagious only after the rash has started and are most contagious for the first 7 to 10 days after the rash appears. After 1 or 2 days, the spots turn into blisters, which fill with pus (forming pustules). After 8 or 9 days, the pustules become crusted. About 30% of people with smallpox die, usually in the second week of the disease. Some of the survivors are left with large, disfiguring scars.

A doctor suspects smallpox when people have the disease's characteristic spots—particularly when there is an outbreak of the disease. The diagnosis can be confirmed by identifying the smallpox virus in a sample that is taken from the blisters or pustules and examined under a microscope or sent to a laboratory for the virus to be grown (cultured) and analyzed.

Prevention and Treatment

Prevention is the best response to the threat of smallpox. Vaccination within the first few days of exposure can prevent the disease or limit its severity. People with symptoms suggesting smallpox need to be isolated to prevent spread of the infection. Contacts of these people need not be isolated because they cannot spread the infection unless they become sick and develop a rash. However, contacts are watched closely and isolated at the first sign of infection.

Vaccination is dangerous for some people, especially those with a weakened immune system. Rarely, even some healthy people have adverse reactions to smallpox vaccination. Adverse reactions are less common in previously vaccinated people than in those who have never received the vaccine. About 1 in every million previously unvaccinated healthy people and 1 in every 4 million previously vaccinated healthy people die from the vaccine. Vaccination before exposure is recommended only for people at high risk of exposure, mainly laboratory technicians and health care workers who handle the vaccine and related materials. After exposure, vaccination can reduce the severity of symptoms. Vaccination up to 4 days after exposure is beneficial but is most effective when given soon after exposure.

Treatment of smallpox is supportive. It includes fluids, symptom relief, assistance with breathing (for example, with a face mask to supply oxygen), and treatments to maintain blood pressure.

Last full review/revision November 2009 by Marguerite A. Urban, MD