  We study an abstract model for the co-evolution between mutating viruses and
the adaptive immune system. In sequence space, these two populations are
localized around transiently dominant strains. Delocalization or error
thresholds exhibit a novel interdependence because immune response is
conditional on the viral attack. An evolutionary chase is induced by stochastic
fluctuations and can occur via periodic or intermittent cycles. Using
simulations and stochastic analysis, we show how the transition between these
two dynamic regimes depends on mutation rate, immune response, and population
size.
