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Yersinia pestis
Yersinia pestis is a bacterium that causes plague (Black Death). Plague is endemic in
rodents such as black rats and prairie dogs. Plague is vectored by the Oriental rat flea,
Xenopsylla cheopis. The flea normally feeds on rats, but will bite humans who live in
close contact when populations are high enough. The Yersinia pestis bacteria multiply
rapidly in the flea, blocking its gut. When the flea tries to ingest a meal it is unable to,
and it regurgitates blood and bacteria into the bite. Plague occurs in several forms, two of
which are important in humans. Bubonic plague victims show symptoms about a week
after being bitten by fleas. Their lymph nodes swell and are called bubos, giving this
form of plague its name. There is ~75% chance of mortality after several days of
headache, chills, and fever. Pneumonic plague is airborne, spread through water droplets
from one victim to another. Victims develop symptoms shortly after infection, with
mortality closer to 90%. Plague has played a major role in human history, with great
global pandemics occurring over 50 years in the 6th century, causing 100 million deaths,
and the famous black death of Europe occurring over five years in the 14th century,
causing 25 million deaths. Plague can be treated with antibiotics, but treatment must
begin shortly after infection for it to be effective.
Oriental Rat Flea, Xenopsylla cheopis
Bubo on neck