There are about 150 species of hairy flies that are known to be parasitic to mammals. Notably there is the Dermatobia hominus, the human botfly. This is going to be nasty folks.
The female botfly deposits their eggs on a living host, a mammal or even another insect which acts as a vector (a method of transmission) to a warm-blooded mammal. Eggs that are laid on a mammal come into contact with the skin and hatch shortly thereafter, fueled by the warmth of the mammalian body. From this hatching they burrow down under the skin where they take residence and grow larger, feeding upon the blood and fluids of the host for months.
Intestinal Invader, Internal Vampire
Some species of botfly reside in the digestive tract of mammals, introduced by the licking of the afflicted sites of botfly egg deposition such as legs or feet, where they act as an irritant. The type of botfly larva such as this is depicted above, resides in the intestines of large animal hosts such as horses and remain attached for nearly a year, feeding until they mature. They eventually release themselves and are passed via elimination to become adult botflies outside of the mammalian body, and the cycle of infestation continues. There are de-wormer medications that can be used to eradicate the botfly larvae in the host animal and hopefully break or interrupt the cycle of parasitism.
These insects in true parasitic form do not kill or actually unduly harm the host, but there are some species of botfly such as in the rodent-infesting variety that seek and consume the testes and ovaries of their host and thus rendering them incapable of reproduction. There is another type of botfly whose presence causes lesions under the skin that become infected by a bacterium they carry. This causes large bumps to form under the skin. Without antibiotic intervention, the host animal usually dies.
American Warble Fly
Growing up in western New York State I have seen what the American Warble Fly (a botfly species member) can do to both livestock and feral game animals. These botflies do not seek out humans as host but do attack larger mammals such as deer and cows. We had a milk cow that had about two dozen large bumps on its back, front shoulders and haunches that we were taught how to remove. Manual palpation via precise squeezing would cause the larval slug to erupt out of the animal’s back in a most disgusting manner! Think big, fat pimple that explodes when squeezed firmly and in the right way. These would come out like explosive child-birth, expelling the still-live white grub. I can still hear those damn things squirt up and hit the low ceiling of the milking parlor with a smack! Yuck!