Fungus gnats!! They infect every single plant indoors. They fly to each plant, lay eggs, eggs hatch and turn into maggots that chew through the roots. Weaking plants and then they get sick and die. You need a few things to get rid of them. First, trap as many as you can with yellow sticky traps layed horizontally on the edges of ALL pots. Then, get some Bt insecticide, the type that will kill the fungus maggots in the soil. Once maggots are mature, you get more tiny flies. The two procedures are the ONLY way to effectivly kill both cycles of these tiny bugs. They also love outdoor gardens and can spell doom for anyone who has a big greenhouse. I use a benefical nematodes as well, and they are watered into my garden soil in summer, to help control the fungus gnats. A few years back, my brussles sprouts were all loaded with these tiny flies. Nothing worse than eating some of the sprouts and seeing tiny dead black flies in the cooking liquid YUK!
The ONLY way to prevent these nasty bugs from getting into indoor plants is to always use STERILE SOIL or sterile seed starting mixes. These mixes contain no bugs or pathogens, and if the gnats are in the plants indoors (even in ONLY one pot) they spread quite fast and you will have ALL indoor plants infected within a few weeks.

Wish I could help further, but I guess no one seems to read my warnings about the kinds of potting soil to use indoors.

I have fungus gnats on my houseplants and indoor herbs as well. They really aren't that big of a deal as long as you make an effort to control the population. I spray periodically with neem concentrate and that seems to keep the population down. It is especially important to spray after watering as that washes the pesticide out of the soil to an extent and the gnats thrive in a moist environment. I always have a few flying around, they seem almost impossible to avoid in any greenhouse or indoor growing environment. I haven't noticed any damage to any of my plants, though.

Baddly infested plants can die or get severely weakend if the gnats are not controlled. I find that the yellow sticky traps are a great help in detecting them as well a strapping many of the flies. I had a single poy with some rose bush cuttings in it. The soil was a prepackaged type and was outdoors in a closed container. Somehow it got infected with the gnats. That was last November, and after a month of seeing many tiny flies indoors, I put out just one 4"x6" stcky trap placed horizontally on the edge of the pot. After a week it had about 20 bugs and after a month over a 100! The damages they do are within the roots of plants where they create a fungus essential to their feeding, and that fungus is not beneficial to the plant roots, so plants can weaken and die.

Thats why i use neem concentrate. It functions as a fungicide as well as an insecticide. Perhaps thats why I have never lost any of my plants to the gnats. They tend to reproduce at a pretty incredible rate. I would imagine that a month of unchecked growth would result in quite a population.

For fungus gnats, try mosquito dunks. Place a dunk in some water and let it sit for a day or two. Use that water to water your houseplants. Do this for a week or two. Works well.