How do fleas spread plague from rats to humans?

Fleas are known vectors that play a critical role in the transmission of plague, primarily responsible for spreading the disease from infected rats to humans. The correct choice emphasizes bite transmission as the primary method of spreading the plague.

When a flea bites an infected rodent, it ingests blood containing the bacterium Yersinia pestis, which causes plague. This bacterium can multiply within the flea's gut, leading to a blockage. When the flea attempts to feed again, it may regurgitate the infected blood into the bite wound of a host, which can be a human or another animal. This direct introduction of the bacteria into the bloodstream during feeding is what facilitates the rapid transmission of the disease.

While other methods, such as direct contact or contaminated food, might also be involved in disease dissemination, they are not the primary means by which fleas transmit the plague. The reliance on the flea's biting action to introduce Yersinia pestis directly into a new host’s system is what makes bite transmission the correct answer in understanding how the plague spreads from rats to humans.