1.   Because unlike the Marmots, the rats had no resistance to the disease that the fleas carried.

2.   It arrived on ships from Asia, carried by fleas that had infected rats on board the ships.

3.   Plague bacteria are carried by fleas and spread from rodents, rabbits and domestic animals to people.

4.   Sylvatic plague, he explained, is carried by fleas, and is the disease known as bubonic plague when it occurs in humans.

5.   It comes from bacteria carried by fleas from infected rats.