These fleas carry the bacteria from an infected rodent and then pass it to humans by biting their skin. In the United States, the bacteria have been linked to prairie dogs, rock squirrels, and other rural rodent populations in the Southwest. Sometimes the plague is spread through contact with infected animals and, less commonly, it is spread from human to human, as in Madagascar. There are actually three types of plague: bubonic, pneumonic, and septicemic.
Bubonic plague is the most common form of plague, and it's the type that’s spread by the bite of an infected rodent flea carrying the bacteria, Hinnebusch says. It causes a fever, headache, chills, and swollen, painful lymph nodes (buboes) that can turn into open sores filled with pus. If it’s not treated rapidly with antibiotics, it can spread in the body. "It can move to the blood, causing septicemic plague, or the lungs, causing pneumonic plague," Hinnebusch says.
Septicemic plague occurs when the bacteria enter the bloodstream, and it causes fever, extreme fatigue, stomach pain, hemorrhaging, and acral gangrene, which is a loss of blood flow and tissue death in the fingers or toes, which can cause the skin to turn black.
Pneumonic plague is the most virulent and serious form of plague. "It's spread from person to person through aerosolized droplets which are breathed in and cause primary pneumonic plague," says Hinnebusch. It can cause fever, headache, and chills, and also rapidly developing pneumonia, chest pain, and coughs that produce mucus or blood. It can develop from a case of bubonic plague or septicemic plague (the latter is very rare), Hinnebusch says, which is called secondary pneumonic plague because it spreads from elsewhere in the body, not from inhaling the bacteria.
While the plague is treatable with antibiotics, it is fatal in 30% to 100% of cases that aren’t treated, according to the World Health Organization.