Do flies get sick? Considering the environment they live in, you'd think they'd be fighting off bugs all the time.
-- Bugged, La Mesa
Heck yes, flies get sicker than dogs. But not very often. It's only logical that any creature destined to spend its life wallowing in garbage, feces, and rotting flesh must be able to handle most of what comes its way. One slow afternoon, when nothing else was going on in the lab, I guess, some scientists decided to count the bacteria residing on a random sampling of garbage-dwelling flies. Average population was 3.683 million per fly, with a high of 6 million. Of course, a bacterium or two will inevitably find it way inside the fly. As will viruses, protozoa, worms, and fungi. Flies not only stomp around in filth, they eat it too. And then regurgitate it, eat it again, and again. Flies are more revolting than you ever imagined.
Just as a nuclear plant worker might get suited up to dig around in a pile of plutonium, our friend the fly is protected from its environment by its hard outer shell, its exoskeleton. The chitin-and-protein armor is tough, resistant to chemicals, and waterproof. That helps reduce the fly's exposure to infectious microorganisms in the first place. But when they are infested, the bug's digestive fluids destroy most of the invaders. Others are ganged upon by cells in the bug's hemolymph (bug blood). Some surround the microorganisms and suck them up; others cluster together and form a sort of permanent capsule around the invaders. This three-layer defense is usually enough to keep a fly feeling fit.
But from time to time, the fly is overwhelmed and heads into a swoon. They're particularly susceptible to fungus infections. Since flies aren't particularly complex organisms, they usually go from a lively musca domestica to musca mortissimo in short order, with only a fleeting period spent as musca not feeling so hot.