Drain flies in showers may be mistaken for worms, explains All About Worms. Drain-flies, also called moth-flies, usually flock around moist areas in a home. The larvae that are spawned resemble worms, but they are most likely insects in the early stages of life.
All About Worms goes on to say that maggots can form within shower drains, and they are most likely the offspring of flies. Maggot larvae tend to be larger than their drain-fly counterparts, and flies breed in unsanitary places. A shower that has not been cleaned in a while is an excellent breeding haven for maggots.
My Get Rid Of Guide categorizes larvae and maggots as sewer worms, although the website concedes they are technically not worms. According to Tiny Worm, small, black worms can also show in toilet bowls and are usually an indicator of insect larvae that have spawned in sewer drains.
In the case of septic systems, the Galveston County Health District of Texas notes that humans can be exposed to parasitic worms in the event of a septic backup, which is why professionals recommend to have septic systems regularly pumped. The odds of actual earthworms being found in septic systems are slim, according to Mother Earth News. Worms can live around septic systems, but they cannot actually live in the tanks, since they need oxygen to survive.