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Smallpox is a potentially deadly infectious disease, now eradicated worldwide after successful global immunization programs. Over the centuries before being eliminated, smallpox devastated populations worldwide.

The World Health Organization (WHO) declared smallpox officially eradicated in 1979. Only a small amount of the virus has been kept alive under tightly controlled conditions in the U.S. and Russia for medical research.

The last case of smallpox from natural transmission was in Somalia in 1977. After eradication, the WHO advised that all countries stop giving routine smallpox vaccinations. Routine smallpox vaccinations stopped in the U.S. and in many other countries in 1972, and in all other member countries of WHO by 1986.

Given the breadth of global vaccinations, many adults living today were likely vaccinated as children.

What Causes Smallpox: The Variola Virus

Smallpox is caused by the variola virus. There are two forms of this virus. The more dangerous form, variola major, caused smallpox disease that was fatal for about 30% of people infected. A virus called variola minor caused a less deadly form of smallpox, with a fatality rate of about 1%.

Research on the variola virus has been tightly controlled by international agreements. A "cousin" virus in the same family as the variola virus, called the vaccinia virus, has been researched more completely and is the virus used to make smallpox vaccine.

How Smallpox Is Spread

Smallpox disease is highly contagious. It spreads from person to person primarily by breathing in the virus, which is transmitted in saliva, during close, face-to-face contact with an infected person.

Smallpox can also be spread by handling the clothes or sheets of an infected person and by directly contacting infected bodily fluids. Very rarely, smallpox has spread among people in small, enclosed spaces, likely via contaminated air through the ventilation system. Animals and insects do not spread smallpox.

Once infected, a person's incubation period -- as the virus moves inside the body's cells, multiplies, and spreads -- ranges from seven to 17 days before symptoms appear. During this pre-symptomatic incubation period, an infected person is not contagious and cannot spread the virus to others.

An infected person is most contagious once symptoms appear and can spread smallpox to others until all smallpox blisters have healed and scabs have dropped off.

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