Fleas harm their hosts, such as dogs, by biting their skin, sucking their blood, and causing them to itch. The fleas, in turn, get food and a warm home. Barnacles, which live on the bodies of whales, do not seriously harm their hosts, but they do itch and are annoying. Usually, although parasites harm their hosts, it is in the parasite's best interest not to kill the host, because it relies on the host's body and body functions, such as digestion or blood circulation, to live. Some parasitic animals attack plants.
Aphids are insects that eat the sap from the plants on which they live.
Parasitic plants and fungi can attack animals. A fungus causes lumpy jaw, a disease that injures the jaws of cattle and hogs. There are also parasitic plants and fungi that attack other plants and fungi. A parasitic fungus causes wheat rust and the downy mildew fungus attacks fruit and vegetables. Some scientists say that one-celled bacteria and viruses that live in animals and harm them, such as those that cause the common cold, are parasites as well.