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Medically reviewed by Last updated on Feb 6, 2024.

What is Smallpox?

Harvard Health Publishing

Smallpox is a contagious and sometimes fatal disease caused by two related viruses: variola major and variola minor. Variola major is the more common and severe form, with an overall historical fatality rate of about 30%. Variola minor is less common and causes a milder form of smallpox that is usually not fatal. Historic death rates were less than 1%. Smallpox eradication was one of the greatest successes of modern public health. Through a sophisticated global vaccination campaign, the World Health Organization officially declared in 1980 that smallpox had been eliminated worldwide. The last known case of smallpox in the United States occurred in 1949, and the last case of naturally occurring smallpox was reported in 1977 in Somalia.

Today, the smallpox virus is known to exist only in secured laboratory stockpiles in the United States and Russia. However, it is theorized that other countries may have possession of the virus as well.

For this reason, there is some concern that terrorists may have access to the virus, which could be used as a bioterrorism agent. Because smallpox has been eradicated, any human smallpox infection would be evidence of bioterrorism. For these reasons, the Centers for Disease Contol and Prevention have developed a response plan for possible smallpox outbreaks, with detailed instructions on how to mobilize appropriate personnel and vaccines.

Smallpox is usually spread through direct and fairly prolonged contact with an infected person, particularly with face-to-face contact. It is typically spread among people who share living quarters. This is probably because patients with smallpox are severely ill in the period when they are most infectious, and so they are unlikely to have contact with many people outside their homes. Smallpox also can be acquired from infected bedding and clothes. Rarely, smallpox is spread through the air of enclosed settings such as buildings, buses and trains.


A case of smallpox passes through six stages.


The diagnosis of smallpox is based on a physical examination and blood tests. Fever and the distinctive, progressive skin rash will indicate smallpox. Your doctor will evaluate your recent health history and symptoms to determine if you have been exposed to and if you have smallpox.

Expected Duration

A case of smallpox usually lasts about 5 weeks. This includes an average of 12 days of the incubation period, 4 days of the initial symptoms, 4 days of an early rash, 5 days of pustular rash, 5 days of scabs, and 6 days for the scabs to fall off.


The only way to prevent smallpox is by receiving the smallpox vaccine, which was developed from the vaccinia virus, which is related to the smallpox virus, but causes a far milder form of disease. In 1972, routine smallpox vaccination was stopped in the United States because the risks of the vaccine itself were felt to be higher than the risk of getting smallpox. The smallpox vaccine caused one to two deaths for every million people vaccinated. Today, because of the possible threat of bioterrorism, the vaccine is being considered for members of the armed services, public health workers, first responders and health care professionals. In addition, the United States government has enough vaccine to respond to a smallpox outbreak in the country. Vaccination within three to seven days after exposure to smallpox may prevent the disease in rare cases, but usually limits its symptoms, and is thought to reduce mortality.



There is no specific treatment for smallpox other than supportive care. Antiviral drugs are being developed and tested for use against smallpox.

Treatment options

The following list of medications are related to or used in the treatment of this condition.

When To Call a Professional

If a single case of smallpox is detected anywhere in the world, there will be a huge media and public health response, which will include very specific instructions about what you and your family should do, where or if to seek treatment, and whether and where you should be vaccinated. If no smallpox has been detected, it is exceedingly unlikely that you would have been exposed and infected with the smallpox virus. It is far more likely that you have a different, more common viral infection. You should call your doctor if you are experiencing any of the symptoms of smallpox infection, such as high fever and rash that first appears as red spots and that are most plentiful on the face, hands, arms, legs and feet.


Historically, the variola major virus, the most common smallpox virus, was associated with death rates of about 30%. It is important to acknowledge that the mortality rates from smallpox infection are historical, and it is not known what effect modern medicine would have on people's ability to survive smallpox. Survivors of smallpox infection usually were left with disfiguring scars on the body, and many developed eye infections that led to blindness.

Additional Info

Centers for Disease Control and Prevention (CDC)

National Library of Medicine (NLM)


Learn more about Smallpox

Treatment options

Care guides

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.