Fleas

Many of the species of fleas that infest domestic mammals and birds will also utilize humans as a host, although people are not the generally preferred host of these blood-sucking parasites.

The most deadly disease that can be spread to humans by fleas is bubonic plague or black death, caused by the bacterium Pasteurella pestis, and spread to people by various species of fleas, but particularly by the plague or oriental rat flea (Xenopsylla cheopis). Bubonic plague is an extremely serious disease, because it can occur in epidemics that afflict large numbers of people, and can result in high mortality rates. During the European Black Death of medieval times, millions of people died of this disease. There have been similarly serious outbreaks in other places where rats, plague fleas, and humans were all abundant. Bubonic plague is mostly a disease of rodents, which serve as a longer-term reservoir for this disease. However, plague can be transmitted to humans when they serve as an alternate host to rodent fleas during times when rodent populations are large. Plague is mostly spread to humans when infested flea feces are inadvertently scratched into the skin, but transmission can also occur more directly while the fleas are feeding, or when a host accidentally ingests an infected flea.

Another disease that can be spread to humans by fleas is known as endemic or murine flea-borne typhus. This disease is caused by a microorganism known as Rickettsia, and is passed to humans by various species of fleas and lice, but especially by the oriental rat flea. Fleas are also the vector of a deadly disease that afflicts rabbits, known as myxomatosis.

Fleas may also serve as alternate hosts of several tapeworms that can infect humans. These include Dipylidium caninum, which is most commonly a parasite of dogs, but can be passed to humans by the dog flea (Ctenocephalides canis). Similarly, the tapeworm Hymenolepis diminuta can be passed to people by the rat flea (Xenopsylla cheopis).