A limited number of bacterial diseases reach their human hosts via an insect intermediary from their main host, usually another species of mammal.
Plague (bubonic plague, the Black Death) has been responsible for the deaths of untold millions of people in terrible epidemics such as the ones that wiped out as much as one-third of the population of Europe in the Middle Ages. It is caused by the Gramnegative bacterium Yersinia pestis, whose normal host is a rat, but can be spread to humans by fleas. The bacteria pass to the lymph nodes, where they multiply, causing the swellings known as bubos. Y. pestis produces an exotoxin, which prevents it from being destroyed by the host's macrophages; instead, it is able to multiply inside them.
From the lymph nodes, the bacteria spread via the bloodstream to other tissues such as the liver and lungs.
Once established in the lungs (pneumonic plague), plague can spread from human to human by airborne transmission in respiratory droplets. Untreated, plague has a high rate of fatality, particularly for the pneumonic form of the disease. Early treatment with streptomycin or tetracycline, however, is largely successful. Improved public health measures and the awareness of the dangers of rats and other rodents have meant that confirmed cases of plague are now relatively few.
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