Although it is essential to consider virulence from the pathogen's perspective, I think Thomas's quote clouds rather than clarifies. Natural selection does not tally the number of pathogens left to die in a dead person. Rather it tallies the copies of genes that make it to the succeeding generation after the immunological battle. To appreciate this point, ask whether our body's immunological defenses are more frightening to us than they are to the pathogens. When pathogens are growing in a host, the death of the pathogens resulting from the exploitation of that host may likewise be favored.
Wills also makes some familiar errors in describing the process of evolution.
He confuses evolution itself with the generation of variation on which natural selection acts and with the reduction in variation that may occur as a result of natural selection. This blurring of evolution with the generation of variation leads Wills to overlook some potentially powerful predictive aspects of current theory that operate at the interface of evolution and epidemiology. The power of the evolutionary approach to epidemiology lies in just such predictions. Particularly harmful diarrheal variants can be expected to expand their territory in areas where pathogens can be transmitted through the water supply. New mild variants are predicted to gain ground in areas that have safe drinking-water supplies.