Fleas are tiny wingless insects that suck the blood of animals, such as dogs, cats, rats, chickens and humans. They live in the hosts' fur or feather, deriving nourishment from their bodies. Swollen flea bites can be very itchy and painful. You can still be irritated by fleas though, even when they don't bite you. Many people are allergic to flea saliva which causes rashes on their skin.