Fleas are a common pest in the UK and beyond and are a parasite of man. They are a bloodsucking insect and a carrier of diseases such as the Bubonic Plague that killed over a quarter of the European population. They are a small, wingless insect with a tough cuticle that bears many bristles and flattened spines. Their size can vary from about 0.1cm to 1cm in length and solely and the feed exclusively on the blood of mammals including humans. There are roughly 2000 subspecies of fleas, however, it is widely distributed with rodents such as rats and mice as they travel across the world on ships. Native species of fleas can be found in tropical as well as polar and other regions around the world.
Infestations of fleas can cause severe inflammation of the skin and rather intense itching. Many animals across the world can build up immunity to the flea bites over a prolonged period, however, humans can occasionally become sensitized after exposure and can develop allergies. Certain fleas feed primarily on rodents or birds but sometimes attack humans, particularly in the absence of their usual host. When the rats were dying of the bubonic plague, the hungry and infected rats were looking elsewhere for a meal. This led them to feast on humans subsequently passing the plague onto them.