Myiasis is the parasitic infestation of the body of a live mammal by fly larvae (maggots) that grow inside the host while feeding on its tissue. Although flies are most commonly attracted to open wounds and urine- or feces-soaked fur, some species (including the most common myiatic flies, the botfly, blowfly and screwfly) can create an infestation even on unbroken skin and have been known to use moist soil and non-myiatic flies (such as the common housefly) as vector agents for their parasitic larvae.

Colloquialisms for myiasis include flystrike and blowfly strike, and the victim or the tissue may be described as fly-blown.