The house fly and other types of “filth flies” can become nuisance pests, but also are important for their potential to harm humans and animals. House flies, for example, can spread diseases such as food poisoning and dysentery. Flies, including stable flies and mosquitoes (which are also classified as flies, or Diptera), can inflict painful bites while feeding on the blood of humans and other animals, and some species transmit disease.

The habits of filth flies favor the spread of bacteria and other disease-causing organisms. Filth flies often feed and lay eggs on garbage, manure and carrion before contaminating human foods and food preparation surfaces by landing on them. When feeding, house flies regurgitate their stomach contents onto food to liquefy it before ingesting it. They also may contaminate food and surfaces by defecating on them.