Red worms in a toilet are usually midge larvae, often called bloodworms. Midges lay eggs on still water, and a toilet that sits unused for a week or more is still water. The larvae are harmless to people, and regular flushing plus a scrubbed bowl and tank usually ends the problem.
Bloodworms get their red color from hemoglobin, which lets them survive in water with very little oxygen. That is why they turn up in toilet bowls and tanks rather than in drains that carry running water. Adult midges get in through open windows, exhaust fans, and plumbing vents, then lay eggs on the water surface.
Bloodworms usually appear after a long absence: a vacation, a guest room nobody uses, or a vacant house during a sale. Check the tank as well as the bowl, because larvae in the tank refill the bowl on every flush.
Two things change the answer. Thin red worms after urinating, or in stool, are a medical question, so see a doctor. And if red worms appear in a toilet that gets flushed every day, water may be sitting somewhere else, such as a sagging section of sewer line that holds standing water.
Nuflow DFW is a licensed trenchless sewer contractor serving 25 Dallas-Fort Worth cities, RMP# 46694. Our drain cleaning clears the buildup larvae feed on, and a camera inspection checks the line for low spots. You keep the footage. Call (469) 701-0597.