"Fleas" is an umbrella term under which many species are listed, because they act very much alike. They are tiny, wingless, bloodsucking insects that live off of other species. The main difference in the types of fleas is in what kind of host species they feast on. Dog fleas, for example, prefer only dogs, although they can transfer from one dog to another by hitching a ride on a human, cat, bird, rat or other warm-blooded animal.
The oldest flea fossils have been found stuck in Baltic amber from the Upper Eocene era (34 to 55 million years ago, the beginning of the age of mammals). It is unknown if fleas were around during the time of the dinosaurs. The oldest surviving flea species is thought to be the snow flea, which has stubby wings and eats resin and decayed plants instead of blood.
There are over 2,000 species of fleas. The most common and most annoying to people with livestock and pets are human fleas (including chiggers), dog fleas, cat fleas, rat fleas, rodent fleas, rabbit fleas and bird fleas. Rodent fleas tend to also like to feast on bird blood and vice versa. All of the fleas will take a bite out of whatever warm body they are on to see if they like the taste of the blood. Just one bite can be painful for the host.
Not only do fleas cause painful bites and blood loss, but they can also transmit diseases. It is thought that the Black Death in the Middle Ages was not caused by rats, but by the rats' fleas (see Resources below). Many animals are allergic to flea saliva and can scratch themselves raw just from a couple of bites.
All flea species are under the scientific insect family name of Siphonaptera. They can be difficult to identify on the body of an animal. The are often teardrop shaped, shiny black, with a tiny head and a large belly (abdomen) and long legs. When popped between the fingernails, they are full of blood. If you find what looks like black dandruff in your animal's coat, brush it out onto a paper towel and then wet the towel. If the black dandruff turns into spots of blood, then it is the flea's droppings. Signs of droppings means fleas are around.
Your livestock, pets, their bedding and your home need to be treated for fleas. This needs to be repeated in order to kill any newly hatched fleas. Although flea adults are relatively easy to kill, flea eggs are incredibly tough. Please see a veterinarian for the safest method of de-fleaing your particular animal.