Plague transmission from these infected animals generally occurs in one of three ways:
- Bites from infected rodent fleas
- Direct contact with infected tissue or bodily fluids
- Inhaling infected droplets (see Plague Transmission for more information).
During an outbreak, Yersinia pestis are able to survive for several months in cool, moist conditions, such as the soil of a rodent hole. Between outbreaks, the bacteria is believed to circulate within populations of certain species of rodents without causing excessive death. Such groups of infected animals serve as silent, long-term reservoirs of infection.
Approximately 10 to 20 people in the United States develop Yersinia pestis infections each year from flea or rodent bites -- primarily from infected prairie dogs -- in rural areas of the southwestern United States. About one in seven of those infected die as a result. There has not been a case of person-to-person infection in the United States since 1924.
Worldwide, there have been small outbreaks in Asia, Africa, and South America. Globally, the World Health Organization (WHO) reports 1,000 to 3,000 cases of plague every year (see Where Is Plague? for more information).
Yersinia pestis infections occur more frequently during spring and summer months, especially in males and people under the age of 20.
Bioterrorism is a real threat to the United States and around the world. Although the United States does not currently expect a plague attack, it is possible that pneumonic plague could be transmitted through an aerosol distribution. The Yersinia pestis bacterium is widely available in microbiology banks around the world, and thousands of scientists have worked with it, making a biological attack a serious concern.