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Symptoms
By Mayo Clinic staff
The first symptoms of smallpox usually appear 12 to 14 days after you're infected. During the incubation period of seven to 17 days, you look and feel healthy and can't infect others.
Following the incubation period, a sudden onset of flu-like signs and symptoms occurs. These include:
- Fever
- Overall discomfort (malaise)
- Headache
- Severe fatigue (prostration)
- Severe back pain
- Sometimes vomiting, diarrhea or both
A few days later, the characteristic smallpox rash appears as flat, red spots (lesions). Within a day or two, many of these lesions turn into small blisters filled with clear fluid (vesicles) and later, with pus (pustules). The rash appears first on your face, hands and forearms, and later on your trunk. It's usually most noticeable on the palms of your hands and the soles of your feet. Lesions also develop in the mucous membranes of your nose and mouth. The distribution of lesions is a hallmark of smallpox and a primary way of diagnosing the disease.
When the pustules erupt, the skin doesn't break, but actually separates from its underlying layers. The pain can be excruciating. Scabs begin to form eight to nine days later and eventually fall off, leaving deep, pitted scars. All lesions in a given area progress at the same rate through these stages. People who don't recover usually die during the second week of illness.
Smallpox vs. chickenpox
In the past, smallpox was sometimes confused with chickenpox, a childhood infection that's seldom deadly. Yet chickenpox differs from smallpox in several important ways:
- Severity and location of lesions. Chickenpox lesions are much more superficial than are those of smallpox and occur primarily on the trunk, rather than on the face, arms and hands.
- Types of lesions. You'll often see a combination of scabs, vesicles and pustules in someone with chickenpox. In smallpox, all of the lesions in a given area are at the same stage.
- Timing of transmission. A person infected with chickenpox can unknowingly transmit the virus to others before symptoms develop. But smallpox becomes infectious only when signs and symptoms appear and remains contagious until scabs fall from the pustules. Smallpox is most contagious after the fever starts and during the first week of the rash. You're less likely to become infected if you're exposed to someone in the later stages of the disease.
- 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.
