Plague is caused by bacteria that are spread from rats to humans by which method?

The bacterium responsible for plague, Yersinia pestis, is primarily transmitted to humans through the bite of infected fleas. Fleas are ectoparasites that infest rodents, particularly rats, and once they ingest the bacteria from their rodent hosts, they can then transmit it to humans. This can occur when fleas bite humans or when infected fleas are in contact with breaks in the skin or mucous membranes. The close association between rats and fleas makes this transmission pathway particularly significant in outbreaks of plague.

The other methods listed, such as ticks, flies, and mites, are not involved in the primary transmission of plague bacteria to humans. While other pests can carry various diseases, the crucial relationship between rats and fleas is what leads to the transmission of plague specifically. This highlights the importance of understanding vector relationships in public health pest management.