Plague is caused by Yersinia pestis bacteria. Blood sucking fleas transmit the bacteria among animals, and various species of rodents can become infected. Human cases are most likely to occur when domestic rats are involved, as these live in close proximity to humans. Sporadic human cases appear after exposure to rodents and/or their fleas (bubonic plague). In cases of primary pulmonary plague, patients become infected by inhaling bacteria-rich aerosols produced by individuals who developed secondary pneumonia in the course of plague severe blood infection.
Following a short incubation period (1-7 days) the patient develops a high fever and a severe blood infection, with a very high death toll, which remains substantial even if appropriate antibiotic therapy is administered. If the patient survives, bubonic plague is characterised by swelling of regional lymph nodes (bubos), which later resolve, and then the patient usually goes on to recover.
Plague prevention is based on general environmental hygiene, with special regards to waste disposal and control of domestic rats.