In Text S1 we also show how excess mortality of a small group of individuals with antiviral treatment rate s y is calculated in a population where the majority of individuals take antiviral drugs at a rate s x . This allows one to determine the antiviral treatment strategy that will evolve at the population level by the process of individual choice. The optimal population and individual rates of antiviral treatment at the population level will be denoted by s Ã pop and s Ã ind , respectively. Figure 5 shows the results of a systematic investigation of the relation between the model parameters and the fractions of individuals taking antiviral drugs (which are determined by the antiviral control rates s Ã pop or s Ã ind ). The top panel shows the fraction of individuals taking antiviral drugs (black lines) and the associated excess mortality (grey lines) as a function of pathogen transmissibility. If transmissibility is low (b,51 (yr 21 )) the pathogen cannot persist, and there is no need to take antiviral drugs. If transmissibility is intermediate there is a positive population optimum (s Ã pop w0) which ensures eradication of the pathogen, while the individual optimum is still zero (s Ã ind~0 ). If transmissibility is high both the population and individual control rates are positive, although eradication is only achieved by the optimal population control rate.