Most of the research on the evolution of parasite traits (for instance virulence), takes for granted that these traits are ‘heritable’ from one infection to the next. In other words, infections caused by a very virulent infection will be very virulent.
In the case of human parasites such as HIV, this is not obvious. In fact, one might argue that most clinicians would expect these traits not to be heritable, that is that the trait should be determined by host factors (genetic and/or environmental).
In 2010, with Swiss collaborators, we showed that it was possible to use a phylogenetic approach to estimate the heritability of the infections traits in the case of HIV. The idea is that a phylogeny of infections reflects the transmission chain (virus sequences that are close in the phylogeny originate from patients that are close in the transmission chain). If the trait is heritable, you expect proximity in the phylogeny to correlate with proximity in trait value. We found that the heritability of HIV set-point virus load, a proxy for virulence, can be high in some populations.
Since then, we have applied this idea to the case of hepatitis C virus infections, where hosts can either clear the virus in a few months or become chronically infected. Here again we found support for a virus control over this categorical infection trait.