The National Academies: What You Need To Know About Infectious Disease

The National Academies

What You Need To Know About Infectious Disease


Smallpox is caused by the variola virus, which emerged in human populations thousands of years ago. The disease comes in two forms—variola major and variola minor. Variola major is the more severe and most common form of smallpox, with a more extensive rash and higher fever. Its death rate is about 30 percent. Variola minor is less common and much less severe, with death rates historically at 1 percent or less.

Usually smallpox is spread through direct and fairly prolonged face-to-face contact but it also can be spread through direct contact with infected bodily fluids or contaminated objects, such as bedding or clothing. In rare cases, smallpox has been spread by virus particles carried in the air in enclosed settings, such as buildings, buses, and trains. Humans are the only natural hosts of variola; no cases of transmission by insects or animals have been documented.

Smallpox outbreaks have occurred for thousands of years but the disease has been eradicated because of a successful worldwide vaccination program. The last case of smallpox in the United States was in 1949 and the last naturally occurring case in the world was in Somalia in 1977.

The first symptoms of smallpox include fever, malaise, head and body aches, and sometimes vomiting. As the disease progresses, a rash develops composed of small red spots that eventually turn into raised bumps. These bumps fill with a thick, opaque fluid and often have a depression in the center that looks like a bellybutton. The bumps become pustules—sharply raised and usually round and firm to the touch. Eventually the bumps form a crust and then a scab, which falls off and typically leaves a pitted scar. A person is contagious until all the scabs have fallen off, which is usually about 3 weeks after the rash first appeared.

There is no specific treatment for smallpox disease. However, a massive program initiated by the World Health Organization (WHO) eliminated all known smallpox viruses from the world in 1977, except for samples some governments saved for research purposes.

The most effective means of preventing smallpox infection is vaccination. Because the virus has been eradicated, vaccinations are not routinely given to the general public as they once were. There is concern that the smallpox virus might be used as an agent of bioterrorism, however. For this reason, the U.S. government is taking precautions for dealing with a smallpox outbreak and, according to the CDC, currently has enough supplies of the smallpox vaccine for the entire population. At this time, however, only military personnel, health care workers, and emergency responders may receive the vaccine.


Explore Other Topics

What do you know about infectious disease?

Which are larger?

  • Correct!

    Bacteria are 10 to 100 times larger than viruses.

  • Sorry, that’s incorrect.

    Bacteria are 10 to 100 times larger than viruses.

Infectious Disease Defined

Peptic Ulcer
An open sore that develops on the inner lining of the stomach, esophagus, or upper small intestine.

View our full glossary