This is an insect known as a "moth fly".  It is a member of Psychodidae, a family in the order Diptera (true flies).  They are frequently found in bathrooms or sinks and the larvae often feed on organic matter found in sink plumbing.  They are sometimes found in the sludge that will clog up sink drains.  Unless the sludge is removed, the flies can persist.  They may also be found in toilets or in moist crevices of shower stalls/bathtubs.  They do not cause any illnesses that I know about and are only an issue as a nuisance.