Flies are harmful to humans, animals, and livestock, and they may infest houses and businesses. House flies are suspected of infecting humans and animals with at least 65 illnesses, including typhoid fever, dysentery, cholera, poliomyelitis, yaws, anthrax, tularemia, leprosy, and TB. Flies regurgitate and defecate wherever they land, mechanically transmitting disease germs. Although flies can transport germs, viruses, and parasites from garbage to our meals, a single touchdown is unlikely to set off a chain reaction that leads to sickness in the ordinary healthy person.