Do you occasionally observe small flying insects that appear to be tiny (1/10 to 1/8 inch long) mosquitos on your houseplants? Fungus gnats are common in houseplant soils with high organic matter that are kept quite moist. They do not pose any problem for humans or pets, other than being annoying, but can harm plants. The adults do not do the damage, rather it is the larvae, or maggots, that cause injury by feeding on the plant roots. Damage can be as dramatic as sudden wilting or just a general loss of vigor, poor growth or yellowing of leaves.
Probably the best control is to avoid overwatering. Let the top inch or so of the soil or potting media dry out between waterings. The soil should be dry to the touch. Unless you completely forget to water your plants, this partial drying will not hurt them. More house plants are killed by over-watering than under-watering. There are chemical controls for fungus gnats. Check with your local garden supply center.