The small "worms" found in your shower drains and tiles are likely not worms at all, but the larvae of the drain fly or moth fly. These larvae may appear brown or black, and upon closer inspection are pale in the middle and darker on the ends.
Cause
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Because drain fly larvae live in and feed on organic debris, they often are found in the slimy film around drains and traps in the bathroom. They are drawn to moist areas frequented by humans.
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Life Cycle
Eggs hatch within two days into the larvae you find in the shower. These larvae live for eight to 24 days before entering the pupal stage for one to two days, then finally become adults.
Check for Breeding Sites
Identifying breeding sites will help you to deal with your infestation. Place a piece of tape face-down over a suspect drain, then check the tape periodically for adult flies, which will emerge and stick to the tape.
Elimination
Once the breeding site is discovered, you can remove or clean it appropriately and therefore prevent the larvae from feeding and reproducing.
Prevention
To prevent drain fly larvae from infesting your household, clean all drains, traps and shower surfaces regularly. Clean pipes when necessary and keep up with septic system maintenance.