Plague is caused by Yersinia pestis bacteria. It can be a life threatening infection if not treated promptly. Plague has caused several major epidemics in Europe and Asia over the last 2,000 years. Plague has most famously been called "the Black Death" because it can cause skin sores that form black scabs. A plague epidemic in the 14th century killed more than one-third of the population of Europe within a few years. In some cities, up to 75% of the population died within days, with fever and swollen skin sores.
Worldwide, up to 3,000 cases of plague are reported to the World Health Organization (WHO) each year, mostly in Africa, Asia and South America.
Plague is primarily an infection of animals including many species of rodents (including mice, rats, ground squirrels, prairie dogs, chipmunks and rabbits). In the United States, it is most commonly transmitted to humans by the bite of an infected rat flea (Xenopsylla species). People are most at risk of infection when they are in areas where these rodents and their fleas are plentiful. Less commonly, humans can become infected in other ways:
When Y. pestis bacteria enter the body through a break in the skin after direct contact with the meat or blood of an infected animal (could happen, for example, when a hunter skins a carcass)
By breathing in droplets of Y. pestis bacteria if a person is in close contact with a human or animal with plague infection of the lungs (pneumonic plague)
From scratches or bites by infected domestic cats
People who are most likely to be infected include hunters, veterinarians, and those who camp or hike in areas where animals are infected with plague. Domestic cats or dogs also can spread the disease to their owners by bringing infected fleas into the home.