Infection and transmission varies with the type of infection. In all cases, plague is caused by a gram-negative bacterium called Yersinia pestis.

Plague is usually transmitted to humans by the bite of an infected rat flea. Rodents carry this organism where it is transmitted to humans via infected flea bites or through the ingestion of flea feces.

Human-to-human transmission is rare except during epidemics of pneumonic plague.