Immune escape at cytotoxic T lymphocyte epitopes. Schematic representation of the takeover of the viral population by virus mutated at a particular CTL epitope escaping the host immune response. Two different epitopes (diamond and circle symbols) are represented with different rates of population turnover. The equations shown represent the dynamics of wild-type (w) and mutated virus (m) as they replicate with different rates (r and r(1 - c)), where c is the ‘cost’ of escape, and the wild-type virus mutates to escape virus at rate μ (note that for simplicity we do not represent the backward mutation). CTL kills wild-type virus at rate k, but does not affect the escape virus.