An Oriental rat flea (Xenopsylla cheopis) infected with the plague bacterium (Yersinia pestis), which appears as a dark mass in the gut. The foregut of this flea is blocked by a Y. pestis biofilm; when the flea attempts to feed on an uninfected host, Y. pestis from the foregut is regurgitated into the wound, causing infection.<unk>  is an infection of the lymphatic system, usually resulting from the bite of an infected flea, Xenopsylla cheopis (the Oriental rat flea). Several flea species carried the<unk> , such as Pulex irritans (the human flea), Xenopsylla cheopis, and Ceratophyllus fasciatus. Xenopsylla cheopis was the most effective flea species for transmittal. In very rare circumstances, as in septicemic plague, the disease can be transmitted by direct contact with infected tissue or exposure to the cough of another human. The flea is parasitic on house and field rats and seeks out other prey when its rodent hosts die. The bacteria remain harmless to the flea, allowing the new host to spread the bacteria. Rats were an amplifying factor to<unk>  due to their common association with humans as well as the nature of their blood. The rat's blood allowed the rat to withstand a major concentration of the plague. The bacteria form aggregates in the gut of infected fleas and this results in the flea regurgitating ingested blood, which is now infected, into the bite site of a rodent or human host. Once established, bacteria rapidly spread to the lymph nodes and multiply. The fleas that transmit the disease only directly infect humans when the rat population in the area is wiped out from a mass infection. Furthermore, in areas of a large population of rats, the animals can harbor low levels of the plague infection without causing human outbreaks. With no new rat inputs being added to the population from other areas, the infection would only spread to humans in very rare cases of overcrowding.
Bubonic plague
Hittite plague
Sylvatic plague
Urban plague
Malaria
Scrub typhus
Pneumonic plague
Flying squirrel typhus