There has been an evolutionary arms race between viruses and potential host cells where host cells constantly figure out new defenses and viruses figure out way

There has been an evolutionary arms race between viruses and potential host cells where host cells constantly figure out new defenses and viruses figure out ways to counter them. The adaptive immune system is a very sophisticated defense mechanism, but not the only one available. E.g. check out Toll like receptors [1], PKR [2], APOBEC3, etc. Viruses of course have to counter these defenses somehow, but it still slows them down, and if only because they require additional RNA/DNA to encode it which can mutate and render subsequent viruses inactive. There is a great lecture on the topic of intrinsic and innate defenses against viruses by Vincent Racianello [3]. [1]: https://en.wikipedia.org/wiki/Toll-like_receptor#Summary_of_known_mammalian_TLRs https://en.wikipedia.org/wiki/Toll-like_receptor#Summary_of_... [2]: https://en.wikipedia.org/wiki/Protein_kinase_R https://en.wikipedia.org/wiki/Protein_kinase_R [3]: https://www.youtube.com/watch?v=QMNbrTlV4_w https://www.youtube.com/watch?v=QMNbrTlV4_w