from Wikipedia:
"While gigantic maggots only eat necrotic tissue in living animals and are thus arguably symbiotic, certain types of maggots are parasitic, such as Botfly larvae. These larvae spend part of their life cycle as parasites under the skin of living animals. As a result myiasis can occur in the host causing symptoms from sores and irritating lesions to death. This is particularly a problem in warm and damp climates where flies multiply more rapidly."
"There are three main fly families causing economically important myiasis in livestock and also, occasionally, in humans:
* Oestroidea (botflies)
* Calliphoridae (blowflies)
* Sarcophagidae (fleshflies)
Other families occasionally involved are:
* Anisopodidae
* Piophilidae
* Stratiomyidae
* Syrphidae
The adult flies are not parasitic, but when they lay their eggs in open wounds and these hatch into their larval stage (also known as maggots or grubs), the larvae feed on live and/or necrotic tissue, causing myiasis to develop. They may also be ingested or enter through other body apertures."