ETIOLOGY AND EPIDEMIOLOGY

By the end of the 18^th century, smallpox had a worldwide distribution, and as recently as 1967, it killed one in four of those infected (World Health Organization [WHO, http://www.who.int/emc/diseases/smallpox/factsheet.html]). With the exception of those who have been vaccinated or previously survived the disease, everyone is susceptible if exposed; mortality is highest in the very young and the elderly.

There are two major forms of smallpox, which differ in their severity. Variola major, the more common (90% of all cases) and serious of the two, has a fatality rate of approximately 30%, while less than 1% of the victims of variola minor succumb to the disease. Two additional forms are known. Hemorrhagic smallpox, which occurs primarily in adults, results in a rash and hemorrhaging into the skin, mucous membranes and the base of the pox lesions. Malignant smallpox, also known as flat smallpox, occurs in 5% to 10% of patients, and is characterized by soft, flat lesions with a buried appearance. These two latter forms of the disease, although rare, are nearly always fatal.

Transmission of the virus is primarily through the respiratory route; infected air droplets are spread during face-to-face contact with an infected person. However, direct contact with bodily fluids and fomites, dust from contaminated clothing and bedding, can also transmit the disease. Although patients are most contagious early in the course of the disease, smallpox can be transmitted until after the lesions scab over and the last scab falls off.

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