> The Old Friends hypothesis doesn't make a ton of sense to me. Viruses did not come around much later than bacteria and parasites, and definitely before the adaptive immune system, so why aren't there "commensal viruses" which fight off parasites for us?
Viruses are even harder to study than bacteria, which are already very difficult because most of them can't be cultured in a lab [1], so we just haven't studied them that much. It's a lot easier to study viruses that cause a disease because we know what to look for. The gut virome [2] alone is likely to contain a lot of commensal viruses that help us fight off parasites but our understanding is in its infancy.
Sorry I edited my comment before I saw your response. I agree in general, and we may find commensal viruses. It's not crucial to the argument though so I removed it.
Viruses are even harder to study than bacteria, which are already very difficult because most of them can't be cultured in a lab [1], so we just haven't studied them that much. It's a lot easier to study viruses that cause a disease because we know what to look for. The gut virome [2] alone is likely to contain a lot of commensal viruses that help us fight off parasites but our understanding is in its infancy.
[1] https://en.wikipedia.org/wiki/Microbial_dark_matter
[2] https://www.thelancet.com/journals/ebiom/article/PIIS2352-39...