Yes. Depends on the virus, but in the case of HIV infection, the patient can produce over 1 billion new virus particles a day. Given the size (10kb) and mutat

Yes. Depends on the virus, but in the case of HIV infection, the patient can produce over 1 billion new virus particles a day. Given the size (10kb) and mutation rate (10-3 to 10-6, iirc) every single nucleotide is mutated every single day. There's more to it than that (lookup "biological filter" for example), but the virus must mutate to avoid drugs, which usually means a fitness hit (lower replication, for example). Upon infection of a new host, the virus can quickly maximize fitness once again. Not a "base version" per se, but a more ideal, fit virus, without many/most of the escape mutations.