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Causes
By Mayo Clinic staffThe variola virus causes smallpox. Once you're infected, the virus replicates inside your cells — first in the lymph nodes and then in your spleen and bone marrow. Eventually, the virus settles in the blood vessels in your skin and the mucous membranes of your nose and throat. When the lesions in your mouth slough off, large amounts of virus are released into your saliva. This is when you're most likely to transmit the disease to others.
How smallpox spreads
Smallpox usually requires fairly prolonged face-to-face contact to spread. It's most often transmitted in air droplets when an infected person coughs, sneezes or talks. In rare instances, airborne virus may spread further, possibly through the ventilation system in a building, infecting people in other rooms or on other floors. Smallpox can also spread through contact with contaminated clothing and bedding, although the risk of infection from these sources is slight.
Types of smallpox
Two main forms of smallpox exist:
- Variola minor. This is a milder form of the disease and causes a less serious illness. It's fatal in less than 1 percent of people who contract it.
- Variola major. By contrast, this form of the disease kills one-third of the people it infects.
Variola major smallpox is further divided into five subtypes:
- Ordinary smallpox. This is the most frequently occurring type, accounting for more than 70 percent of all smallpox cases.
- Modified smallpox. This milder form of the disease occurs in people who have been vaccinated against smallpox in the past.
- Variola sine eruptione. People with this form of the disease develop fever but no rash.
- Hemorrhagic smallpox. This rare form is characterized by a red, pinpoint rash and bleeding in the skin and mucous membranes. In some cases, hemorrhagic smallpox may destroy the entire skin surface and all mucous membranes. Hemorrhagic smallpox is almost always fatal within three to four days.
- Flat smallpox. This rare form, which occurs mainly in children, also is often fatal. The early signs and symptoms are similar to other forms of the disease, but the lesions are flat and never become filled with pus. Eventually, the skin takes on a rubbery appearance. Bleeding in the skin and intestinal tract also may occur.
- Smallpox. Centers for Disease Control and Prevention. http://www.bt.cdc.gov/agent/smallpox/index.asp. Accessed June 17, 2009.
- Hildreth C. Smallpox. Journal of the American Medical Association. 2009;301:1086.
- Friedman HM, et al. Smallpox. http://www.uptodate.com/home/index.html. Accessed June 17, 2009.
- Smallpox. World Health Organization. http://www.who.int/mediacentre/factsheets/smallpox/en/. Accessed June 17, 2009.
- Taub D, et al. Immunity from smallpox vaccine persists for decades: A longitudinal study. The American Journal of Medicine. 2008;121:1058.
- Rotz L, et al. Smallpox and bioterrorism. In Mandell, Bennett, & Dolin: Principles and Practice of Infectious Diseases. 6th ed. Philadelphia, Pa: Elsevier; 2005. http://www.mdconsult.com/das/book/body/143878763-2/0/1259/0.html. Accessed June 17, 2009.
- Varicella disease questions & answers. Centers for Disease Control and Prevention. http://www.cdc.gov/vaccines/vpd-vac/varicella/dis-faqs-gen.htm. Accessed June 18, 2009.
