Houseflies are major pests. If it has recently walked in excrement, it may transmit pathogens causing typhoid, cholera, dysentery, leprosy, poliomyelitis, and infectious hepatitis, as well as the eggs of parasitic worms. Many hundreds of thousands of people living along Africa's rivers are permanently blinded by small roundworms introduced by the bite of the blackfly. Flies also spread disease and parasites to animals. Depending on the type of fly, the larvae remain in the skin and cause boil-like lesions, or they wander extensively through the body and cause damage to various organs.
Intestinal myiasis occurs when humans ingest food containing the eggs of larvae.
Each adult female fly lays several hundred eggs in decaying vegetable matter, including manure, compost, and garbage. The larvae, or maggots, may complete their growth in as little as a week, with five more days spent in the pupae stage. The pupae is defenseless but generally escapes predators because it is dark brown and buried in soil or heaps of waste. Six or more generations may hatch in a single summer, resulting in a VAST number of flies. Extensive use of insecticides has caused many housefly populations to develop resistance to chemicals that formerly killed them.