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Germs that infect humans

Variola virus

Variola virus

© Robert Alain, SME, INRS-Institut Armand-Frappier

Microorganism: variola virus, a member of the Poxviridae family

Disease: smallpox

Occurrence of the disease

History: in the process of colonizing the Americas, Europeans brought with them many diseases unknown to the aborigines, including smallpox. At that period, smallpox was one of the most widespread diseases in Europe, and Europeans had developed some degree of resistance to it. Aborigines, on the other hand, had no resistance to the disease and were devastated by its introduction in the New World. In 1966, the World Health Organization (WHO) undertook a worldwide vaccination campaign that succeeded in eradicating the disease eleven years later in 1977. Nevertheless, a laboratory accident caused the death of one person in 1978.

Current situation: the smallpox virus now exists only in a few laboratories around the world where it is kept for research purposes.

Forecast: the destruction of the last colonies of variola virus is planned for the year 2002.

Symptoms of the disease: smallpox is a viral disease that causes a severe skin rash in the form of pustules. Other symptoms may occur, such as fever, headache, back pain, and occasionally abdominal pain and vomiting.

There are two types of smallpox: variola minor and variola major. Variola minor has a mortality rate of less than one per cent; that of variola major is 20 to 40%, or more.

Incubation period: the incubation period varies between seven and 19 days.

Contagious period: the contagious period starts with the first lesions and lasts until the disappearance of the last scabs (about three weeks).

Transmission: person to person through ambient air

Hosts: humans are the only host for this disease. Today, the last colonies are stored in freezers under high security.

Geographical distribution of the microorganism: this organism used to be found worldwide. However, no case of smallpox has been reported since 1977.

Vaccine: Jenner developed a vaccine against smallpox in 1798. Mass immunization with the cowpox virus (a similar virus occurring among cattle, but not infectious to humans) has permitted the eradication of smallpox.