Four Conceptual Comments
1. According to the model just described, a disease can change its virulence even if the different strains of the disease never change their levels of virulence. This is possible because the virulence of a disease is an average of the virulence of the different strains, and strains can change their frequencies.
2. Although the model concerns the evolution of a virus’s virulence, the model focuses on counting hosts, not counting viruses. This point was visible from the start, when R was defined. The R value of a viral strain is the average number of people that a person housing the strain infects, and a change in strain frequency occurs when one strain has a higher R value than the other. We’re not counting the number of low- and high-virulence virus particles and comparing them; we’re counting the number of new hosts that have each infection and comparing them. To visualize this point, consider a simple example. Suppose that one viral strain has an R value of 3 while a second has an R value of 2, but the hosts housing the former strain have a much smaller number of virus particles inside them than the hosts housing the latter.
This point has an analogue in the biological and philosophical literature on the units of selection problem. To conceptualize group selection, you need a notion of group fitness. The question is whether the fitness of a group of organisms is measured by the number of individual organisms the group produces, or by the number of new groups that the group founds (Okasha ). Discussion of group selection has mostly focused on the evolution of altruism and selfishness, which are traits of individual organisms, so the first measure of group fitness is the one that usually gets used (Sober and Wilson ). Even so, the second measure is interesting and may have important applications.
3. The model I’ve described provides a lesson concerning the following line of reasoning: ‘If a virus is maximally virulent, it will kill its host before the virus has a chance to infect new hosts. Therefore, an infectious disease must evolve in the direction of reduced virulence.’ This argument is fallacious because it overgeneralizes. It’s true that a maximally virulent virus will promptly disappear from the host population. However, it does not follow that a higher-virulence strain will automatically be replaced by a strain of lower virulence when the higher-virulence strain is less than maximally virulent. The mistake involved here might be called the fallacy of fixating on the most extreme case.
4. It’s one thing to describe what it takes for the virulence of an infectious disease to increase or decline; it’s something quite different to predict what will happen in a given epidemic. The reason for this gap is that the values of D, E, and F for the different strains of an infectious disease can be hard to estimate, and those values can vary in space and time, so that good estimates for here-today may be way off when it comes to there-tomorrow.