There are approximately 60 species of fleas in the UK, all of which are bloodsucking parasites of warm-blooded animals.

Fleas are dark brown, flattened laterally (site to side) and covered with backward pointing "combs" of bristles. This allows them to travel forwards very rapidly between the hairs of their host animal and also helps them to cling tightly and resist dislodging by scratching. Fleas have well developed back legs, enabling them to jump onto the host and also to escape if necessary.

The main concern is the spread of disease. Two disease in particular, Murine typhus and bubonic plague, each oif whci are zoonoses (animal disease spread to man through flea bites). The Tropical Rat Flea Xenopsylla cheopis, from the Black Rat spread plague in the 14th and 17th centuries, killing one third of the population of Europe during the Black Death.

Fleas can also transmit tapeworms - the eggs being swallowed by the host when biting the fur in an attempt to kill the flea. Flea bites irritate and can sometimes produce walnut-sized wellings. Some people become sensitized to the bite. Scratching can lead to secondary infection. Fleas are unwelcome residents in houses that keep a pet dog or cat. They are also a problem in hospitals where feral cats live in ductings.