Any good (read successful) parasite will kill their host slowly. It will rob the fish of nutrients making them very weak and susceptible to other diseases. The fish will eat more and more (like a person with a tapeworm) until the parasite blocks their intestine. Then the fish dies, the parasite wriggles out and makes lots more parasites. I had a lot of luck with the jungle food, but I kept on having my fish get re-infected until I bought the Gel-tek anti-parasite food and put it on algae wafers (10 drops, 5...let dry...5 more...let dry) so that I could get rid of the worms in the pleco.