Biology of Smallpox

Pustules on a smallpox-infected boy. (Smallpox Disease Images, n.d.)

Smallpox is an infectious disease caused by the Variola virus of the genus Orthopoxvirus. There are two strains; the more common and more severe is Variola major; the less common and less severe is Variola minor. V. major can cause four forms of illness: ordinary, which accounts for about 90% of cases; modified, which is a mild version of “ordinary” and occurs when previously vaccinated people get Smallpox; and flat and hemorrhagic, which are both very rare and very severe forms. V. minor is less severe than V. major and only has death rates of 1% or less. Humans are the only known host of the variola virus. (Smallpox Disease Overview 2004)

                Any form of Smallpox may be fatal, but overall the death rate is as high as 30%, according to the CDC (Questions and Answers 2009).

                Smallpox is most often spread directly from person to person by contact or through contact with infected bodily fluids. It may be spread less directly through contact with items that have become contaminated, such as clothing. Smallpox rarely spreads through the air. (Questions and Answers 2009)

                A Smallpox case may last 29-41 days if the infected person survives. There is an incubation period of about 7 to 17 days, during which time people are not contagious. The first symptoms are a high fever, aching, and vomiting; these usually leave people too sick to move around and last for 2 to 4 days. After this comes a rash that starts on the tongue and in the mouth but spreads all over the body, concentrating on the head and legs. The most contagious point is when the sores in the mouth burst open, spreading the virus into the mouth and throat. About 24 hours after the rash appears, the fever drops and the person may begin feeling well. The rash will raise into bumps by the third day and by the fourth those bumps fill with pus. At this point the fever returns and remains high until scabbing occurs. Around the fifth day after the rash appears, the bumps become round and hard (pustules). About five days after the formation of the pustules, they begin to scab. Most sores will be scabs by 14 days after the appearance of the rash. Once the pustules scab, they begin to fall off. As the scabs fall off, they leave scars on the skin. Most scabs fall off about a week after they form. Until all the scabs have fallen off, a person is still considered contagious. (Smallpox Disease Overview 2004)

                Though there is no treatment for Smallpox, there is evidence that vaccination within the first 4 days of exposure “could abort or significantly reduce the severity of Smallpox disease” (Questions and Answers 2009).

                Prevention includes getting a vaccination and avoiding infected people. However, the vaccination does not provide lifelong immunity, and the first vaccination has the side effect of a rash that is similar to, but should not be confused with, the rash resulting from Smallpox disease. (Questions and Answers 2009).

Comparison of rash clustering on a smallpox patient and a chickenpox patient. (Smallpox Disease Overview, 2004)