Evolution of parasite into ideal parasite?

Question

An ideal parasite is a parasite which thrives within a host without harming it (according to my NCERT textbook). \n\nThen, quite obviously, why hasn't that happened yet? I mean, natural selection can lead to such a species, which is beneficial to us and also, not at all detrimental to the parasite.

Answer 1

You may be thinking of mutualism or commensalism, types of symbiotic relationships where an organism benefits from its host and either 1) benefits the host as well (mutualism) or 2) doesn't affect the host (commensalism). An example of mutualism would be our gut microbiome, which benefits from the shelter and nutrients we have in our intestine while helping us digest food and protect against diseases. You could argue that by using some of our nutrients the gut microbiome is causing harm, and indeed some of the organisms can become pathogens when the conditions are right, but the benefit usually outweighs the harm enough for this relationship to be considered mutual.

The definition of a parasite, on the other hand, is an organism that benefits from a host while harming them. Different parasites will harm their host to different extents: a tick will take a small amount of blood while causing inflammation and irritation, while the ebola virus will often kill the host. It is often beneficial for a parasite to leave its host alive, so that the host can continue benefiting it. One example of evolution in this direction is the SARS-CoV-2 virus, made famous by the COVID-19 pandemic, which has evolved to become less lethal over time. The current SARS-CoV-2 strains could then be considered closer to the "ideal parasite" than earlier variants, but any true "ideal parasite" would really be commensal or mutualistic.