  Co-evolution of two coupled quasispecies is studied, motivated by the
competition between viral evolution and adapting immune response. In this
co-adaptive model, besides the classical error catastrophe for high virus
mutation rates, a second ``adaptation-'' catastrophe occurs, when virus
mutation rates are too small to escape immune attack. Maximizing both regimes
of viral error catastrophes is a possible strategy for an optimal immune
response, reducing the range of allowed viral mutation rates to a minimum. From
this requirement one obtains constraints on B-cell mutation rates and receptor
lengths, yielding an estimate of somatic hypermutation rates in the germinal
center in accordance with observation.
