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.

I will try explain in my way, Consider there is a community of few conpletely disease free humans . Can you Think what major advantage it will give to them if resources to them are well supplied constantly ! They will reproduce on there maximum potential and the population will burst out since there will be no external force to keep a check in the drastically growing population of those disease free humans they will overexploit the reaources and exhaust all the resource. Hence the harmful parasite co-evolved with the humans just to counteract their immunity against parasite so that in another way they can keep a check make sure yhis doesn't happen in future to prevent the nature's other species from being destructed .

Hope i was able to explain you 😬

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Yes because death of host would be lead to death of parasite therefore an ideal parasite should be able to harm the host without killing it .

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