Fleas are parasites that feed on blood. Fleas can live on any warm-blooded animal, but seem to prefer humans, cats, dogs, opossums, rats and other rodents. They can also be found on shoes, pant legs, or blankets. Fleas were known for spreading the Bubonic Plague but they also transmit the bacterial disease murine typhus to humans through infected rats. Their saliva is an allergen that can cause allergic reactions in pets and humans. Fleas can also transfer tapeworms and cause anemia in pets.