Cause of Smallpox

An infection with a double-stranded DNA virus known as the variola virus is the cause of smallpox. The variola virus only infects humans. However, because the smallpox vaccine has been so successful, the disease has virtually been eliminated throughout the world.

What Causes Smallpox?

The cause of smallpox is an infection with the variola virus. This is a double-stranded DNA virus in the family Poxviridae and the genus Orthopoxvirus. Variola virus only infects humans.
Thanks to the success of vaccination, the last natural outbreak of smallpox in the United States occurred in 1949. By 1972, routine smallpox vaccinations for children in the United States were no longer needed. In 1980, smallpox was said to be eliminated worldwide, and no cases of naturally occurring smallpox have happened since.
Because smallpox was wiped out many years ago, a case today would be the result of an intentional act. A single confirmed case of smallpox would be considered an emergency.

How Is Smallpox Transmitted?

Smallpox is highly contagious. In most cases, smallpox transmission occurs by inhaling droplets of saliva, which are full of virus, during face-to-face contact with an infected person. Usually, fairly prolonged face-to-face contact (three or more hours) is required.
Occasionally, smallpox can be transmitted through:
  • Direct contact with infected fluids or contaminated objects
  • The air.
When the smallpox virus enters the body, it spreads within the bloodstream to small blood vessels in the skin. After 7 to 17 days, smallpox symptoms will begin.

Smallpox Disease

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