From fossil remains in ancient Egypt, we know that the common bed bug, Cimex lectularis, has troubled humans in their sleep for at least 3550 years. Both males and females of this nocturnal species feed exclusively on animal blood. Besides humans, bed bugs bite dogs, cats, birds, rodents and bats. They are not known to transmit diseases, but as they feed, bed bugs inject a substance that can cause irritation, welts, swelling and extreme itching in sensitive individuals.