That would not be a good assumption. An already ill fish may not show any signs. The parasite or bug may infect the tank. As many illnesses of the Aquatic nature are not always visible. Carefully checking your tank and existing fish after a new fish is introduced and dies is the best practice. Immediate water change and maybe a little medicated precaution, should be the next move. Not good to just let it go. Yes, fish die, but really it is not the fish that is at fault for that. Especially not one in captivity.