According to Florida State University's Molecular Expressions Microscopy Primer, house flies have sponge-like mouth parts and cannot bite. Many other types of flies have piercing mouth parts including stable flies, who live on the blood of livestock, pets and people. Horse flies also may bite using scissor-like mouth parts.
While they do not bite, house flies are a health concern in other ways. House flies derive food from many bacteria-laden items, such as rotting garbage and animal carcasses. When they land on a food source, their feet become contaminated. Later, when the fly lands on a person or piece of food, it transfers some of the bacteria, spreading pathogens and disease.