Plague is usually acquired from the bites of fleas infected with the bacterium Yersinia pestis. Fleas become infected by feeding on rodents, such as chipmunks, prairie dogs, ground squirrels, mice, and other mammals that are infected with the Yersinia pestis bacterium. These fleas can then transmit the bacteria when they subsequently bite humans. Less commonly, plague can be acquired from being bitten or scratched by infected animals or by handling carcasses of animals (often rodents or rabbits) that had been infected. Persons can also become infected by inhaling respiratory droplets from a coughing person who has plague pneumonia. Although a handful of people in the western United States become infected each year, plague has never been reported in Wisconsin.
The typical sign of the most common form of human plague is a swollen and very tender lymph node, accompanied by pain. The swollen node is called a "bubo" (hence the term "bubonic plague"). Besides the swollen lymph node, bubonic plague symptoms include fever, chills, headache, and extreme exhaustion. A person usually becomes ill with bubonic plague 2-6 days after being infected. When bubonic plague is left untreated, plague bacteria invade the bloodstream. When plague bacteria multiply in the bloodstream, they spread rapidly throughout the body and cause a severe and often fatal condition.
The other form of human plague is called pneumonic plague. This is an infection of the lungs with the plague bacterium, causing a very severe respiratory illness.