Do u believe that an ideal parasite should be able to thrive within the host witout harming it?

then why didnt natural selection lead to the evolution of such totally harmless parasites?

In parasitism one of the partners is benefited because it resides outside or inside the body of the host and gets free accommodation and food while the host is affected due to loss of nutrients. Parasites are always harmful. 

There are organisms which thrive within the host without harming it, but they are not called ideal parasites. The term for such interaction is commensalism example includes gut bacteria inhabiting gut of humans. Thus, you can say that natural selection has indeed lead to the evolution of organisms which are not harmful to their hosts. But, they won't be called as ideal parasites. 

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