What is the cause of plague transmitted from rats to humans?

The cause of plague transmitted from rats to humans is primarily due to fleas, specifically the oriental rat flea (Xenopsylla cheopis). These fleas are often infested with the bacterium Yersinia pestis, which is the causative agent of the plague. When a flea bites a rat that is infected with Yersinia pestis, the bacterium can multiply within the flea. If the flea then bites a human, it can transfer the bacterium, leading to infection.

Plague transmission can occur through other means, such as direct contact with infected animals or through respiratory droplets from a person infected with pneumonic plague, but in the context of transmission from rats, fleas are the primary vector. This highlights the importance of controlling flea populations in areas where rats are prevalent to decrease the risk of plague outbreaks.