Fruit flies are annoying. All you wanted to do was leave your fruit out on the counter during the hottest months of the year — or perhaps just wait two or three days before washing your dishes in peace, like a normal person — and instead fruit flies begin feeding on your rotting produce and having little dance parties in your sink drain and your fruit bowl, and making more and more (and more) of themselves. It doesn't take long. Soon there's a cloud of tiny, sluggish, hovering insects creating a sort of haze at eye level in your kitchen, and you, my friend, have a fruit fly infestation. But do fruit flies care about you and your convenience or sanity? Absolutely not. And they can get into your home through just a few small holes in the screen door.
In their defense, fruit flies (genus Drosophila) pose very few health risks. Unlike mosquitoes, they don't bite or spread disease. In fact, they're the reason we know as much about the science of genetics as we do. Fruit fly generations turn over so quickly, they've been invaluable test subjects for scientist studying genetic evolution — with fruit flies they were able to observe in 30 years what it would take around 200 years to see in mice!
However, even if you can respect a fruit fly's contributions to science, it's likely that doesn't make it less annoying when you accidentally inhale one up your nose while you're loading the dish washer.