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Health A-Z

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Harvard Medical School

What Is It?

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.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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