Some parasite eggs can be expelled in the feces, settle in the gravel, hatch into a free-swimming stage, and infect other fish. Don't share any water/equipment between this tank and your healthy ones.
Also, the type of medication depends on the parasites involved. Anything with praziquantel will do cestodes and trematodes (tapeworms and flukes respectively), fenbendazole or levamisole treat nematodes (roundworms), and metronidazole is an antibiotic generally used to treat protozoal infections. Since you don't know what it is, go for a parasite remedy that contains several active ingredients from the list above. I usually make my own since there are none currently available that are comparable to the food that Jungle Labs used to make - it contained prazi, levamisole, and metro all in one).