Fig. 2.

Expected proportions and absolute numbers of respiratory droplet transmissible A/H5N1 virions within a host initially infected by strains that require five (blue), four (green), three (orange), two (red), or one (purple) mutation(s) to become respiratory droplet transmissible, calculated from the deterministic model. A. Expected proportion of respiratory droplet transmissible A/H5N1 viruses in the total virus population over time in the random mutation case (when all mutations are fitness neutral). B. The absolute number of respiratory droplet transmissible A/H5N1 viruses in a host; the intersections with the grey line indicate the point when at least one virus in each host is expected to have the required mutations. The change in slope is due to the transition in the virus population from exponential expansion to constant size.