Zoom in on microorganisms

Germs that infect humans

Yersinia pestis

Yersinia pestis

© Dennis Kunkel Microscopy, Inc.

Microorganism: the bacterium Yersinia pestis

Disease: plague

Occurrence of the disease

History: during the Middle Ages, this disease was known as the Black Death, because of the black stains it produced in the skin.

Current situation: in the United States, this disease is rare (around 25 cases per year) and the mortality rate is 15%. Canada has not had a single case of plague for many years.

Mechanism of action of the microorganism: the bacteria multiply in the blood and block the natural defenses of the infected person.

Symptoms of the disease: the first symptoms of plague are non-specific. These include fever, muscle aches, chills, nausea, sore throat, and headache. As the disease progresses, there is swelling of the lymph nodes. These swellings are known as buboes. Subcutaneous hemorrhages appear, causing black patches on the skin. Untreated plague is fatal in 50 to 60% of cases, within three to five days. In some cases the microorganisms may invade the lungs.

Incubation period: from one to seven days

Contagious period: under appropriate conditions, fleas can transmit the disease over the course of months.

Transmission: this disease is spread by rodents and their fleas; the fleas can spread the disease to other animals, including humans. The microorganism is actually transmitted through the bite of an infected flea.

Hosts: wild rodents such as squirrels. Domestic cats may sometimes be a source of infection.

Discoverer of the microorganism: Kitasato and Yersin in 1894

Treatment: certain antibiotics such as streptomycin or tetracycline

Geographical distribution of the microorganism: most mountainous and rural high-altitude regions of Africa, Asia, and South America.

Prevention: vaccine

Vaccine: there is a vaccine made from killed bacteria; it is administered in three doses.

In regions where the disease is endemic, vaccination may be recommended if direct contact with rodents might occur.