Fleas are wingless insects that live on animals including humans. They feed on the blood by biting the animal and live in the fur or hair of the host. There are many different types of fleas but they all belong to the order Siphonaptera. Fleas are very small (1/16th-1/8th inch) and can jump from animals to humans. The most common way for a patient to be infected is by having a pet with fleas. Generally flea bites cause red itchy bumps. At times these bites can become infected with bacteria causing a skin infection called cellulitis. The bite of fleas can also carry diseases that are passed on to patients through the saliva of the flea during the bite. The most well-known disease is bubonic plague caused by the bacteria Yersinia pestis.