The three different species of fleas that mostly attack humans are cat fleas, dog fleas and human fleas. These are tiny, wingless insects which are parasitic in nature. They feed themselves on the blood of humans and animals for survival. To obtain the food, they pierce their sharp jaws into the skin and suck blood from the body. Fleas are harmful to humans because they live on a number of hosts, and as a result, they can transmit diseases from one host to another.