We calculated the mutation frequency occurring during these five chronic infections, each caused by a distinct HRV serotype and for various periods of time. Mutation frequency was different in each host and likely reflects different viral replication levels, different host environments, and different durations of infection. The calculated mutation frequency ranged between 7.27610 206 and 3.88610 205 change/nt/day. Recent data on HRV-39 genome evolution over a 5-day acute experimental infection in human immunocompetent volunteers revealed a mutation frequency of 3.4610 24 change/nt (equivalent to 6.83610 25 change/ nt/day) [37] . Therefore, the HRV mutation frequency observed in some immunocompromised hosts infected over months was of the same order of magnitude as that observed in acute 5-day infections.