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Why is pneumonic plague more contagious than bubonic plague?
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plague
Image depicting victims of the sixth biblical plague.
Pneumonic plague is significantly more contagious than bubonic plague because of its mode of transmission. While bubonic plague is not directly infectious from person to person—relying instead on infected fleas to transmit the bacterium Yersinia pestis from rodents to humans—pneumonic plague allows for direct human-to-human transmission.1
In cases of pneumonic plague, the lungs are extensively invaded by the bacteria. This enables the pathogen to be spread to other people through infectious droplets expelled when an infected individual coughs or sneezes. Because this form of the disease can be transmitted via respiratory droplets, it is highly infectious, whereas bubonic plague typically requires a vector, such as a flea bite, to spread.1