Fleas (order Siphonaptera) cause itchy papules, which are often in groups of three, the so-called breakfast, lunch and dinner configuration. Human fleas, Pulex irritans, are relatively uncommon. The most likely species to bite humans are the cat flea, Ctenocephalis felix, or the rat flea, Xenopsylla cheopis. Fleas are known to transmit the bacteria that cause plague, although this is rare in North America.

Fleas are bloodsucking parasites of pets, livestock and humans. These are laterally flattened and wingless insects. Fleas can travel rapidly by jumping, using their legs and a spring-like mechanism in the body. They are capable of spectacular leaps, covering distances up to one hundred times their body length.

Only the adult stage feeds on blood, which they obtain with their piercing/sucking mouthparts. Flea larvae feed on organic debris in bedding or sleeping areas, an in dust and lint debris in carpeting. However, they are most common in areas where animals sleep. Female fleas lay eggs in these areas. The eggs take about two weeks to hatch, depending on the temperature. Larval fleas look very different from the adults, having long body hairs, and appearing caterpillar- or worm-like, without eyes or legs. Larvae spin a silken cocoon before transforming into adults. The period of a complete lifecycle from egg to egg is 3-4 weeks depending on food availability and temperature. The warmer the temperature the faster the lifecycle.

Adult fleas are capable of living long periods without feeding. In a few instances fleas have been known to live on or two years without a blood meal.

Fleas are responsible for a number of medical and veterinary problems. Flea bites themselves are irritating and can become swollen or inflamed. Scratching can lead to secondary infection. Some flea species are known to transmit diseases, such as plague and murine typhus, and parasites, like the cat tapeworm. Additionally, many pets develop allergic reactions to flea bites.

The most common flea found on household pets, such as cats and dogs, is the cat flea, Xenopsylla cheopis. The human flea, Pulex irritans, is rarely encountered in California.

Control of fleas involves killing adults on infested pets, and destroying eggs, larvae and adults in bedding, carpets, and furniture. This could involve treating both the pet and home simultaneously with insecticides, and frequent washing of bedding and keeping carpets as clean as possible. Infested furniture, such as couches or overstuffed chairs should be discarded. Adults can live for long periods of time without feeding, particularly at cold temperatures. It should be noted that there is little evidenced that feeding pets yeast, vitamin B12 or garlic will prevent flea infestations.