Question

More vulnerable to predation by making it physically weak. Do you believe that an ideal parasite should be able to thrive within the host without harming it? Then why didn't natural selection lead to the evolution of such totally harmless parasites?

Answer 1

First of all, if an organism (parasite) is able to derive benefit without harming the host, it would be considered commensalism the very instant instead of parasitism.

Secondly, since nature has various methods to keep different organism populations in check, it is also one such method, and a parasite not harming the host would not fulfil the nature’s task.

Third, if the given organism is a parasite, then it is supposed to derive nutrition from the host, and that is not possible without affecting the actual functioning of the host's system.