The New Science of Darwinian Medicine
I have had discussions about this phenomenon, and it appears our ideas are not quite current. From New Physician magazine, Randolph M. Nesse, MD, co-author of Why We Get Sick: The New Science of Darwinian Medicine:
Bacteriologists used to think that , over time, pathogens evolved to reduced virulence, since the pathogen was seemingly dependent on the continued survival of its host. If the host dies, the pathogen dies, so reduced virulence would benefit the pathogen—or so one would think. Proponents of this view did not fully understand that natural selection does not select for genes that increase cooperation in nature or even survival per se. Instead, as emphasized by biologist Paul Ewald, it selects for genes that increase reproductive fitness. In otherwords, a pathogen is shaped to whatever level of virulence that maximizes its own reproduction.
If a cold virus makes you so sick you die or that you can’t leave the house to transmit virus to other people, then natural selection therefore selects for cold viruses that do not incapacitate/kill us [like the classic thought]. On the other hand if pathogen transmission does not depend on host mobility, no such selection for reduced virulence would take place. For example, because cholera is transmitted through the diarrhea it produces, a victim sick in bed can still spread the virus if the diarrhea can get into the water supply. In this case, the pathogen’s transmission depends not on the hosts’s mobility, but on the volume of diarrhea produced, so strains making people sicker spread faster. The same is true for E. coli infections in a hospital. They are usually transmitted on the hands of medical staff, a circumstance leading to the evolution of increased virulence.
To get a better idea of this, consider some of Ewald’s research into the cholera epidemic that has been unfolding over the past decade in S. America. In areas where public sanitation is good, the cholera bacterium spread only if people are up and around [and not washing their hands], so natural selection should lower cholera’s virulence. However, where water systems are contaminated by sewage, strains that produce more diarrhea should reproduce faster. Sure enough, Ewald has discovered that in areas without modern sanitation, selection is making cholera more dangerous as measured by the amount of [diarrhea-causing] toxin produced by the bacteria.