Thanks to everybody for a lively discussion of the evolution of virulence today; pictured here is a rabbit victim of myxoma virus, the classic example used to promote the idea that pathogens will evolve towards a more benign state.

Key points from today:

1) Pathogens do not always evolve towards commensalism.

2) Pathogens that exclusively use vertical transmission – mother to baby – will evolve to a more benign state than those transmitted horizontally.

3) The mode of transmission matters: directly transmitted pathogens are less virulent than vector borne pathogens, and environmentally (or waterborne) illnesses tend to be the most severe.

4) Opportunities for transmission affect pathogen evolution – increased transmission selects for more virulent pathogens with a decreased duration of infectivity (and greater mortality). Decreased transmission selects for less virulent pathogens.

5) Competition between pathogens in a single host or single population can select for increased virulence.

6) Humans may be able to affect the evolution of pathogen virulence with some public health measures – but the definitive test awaits!

The writing project due next tuesday is as follows: Suppose the New Mexico Legislature is taking up the following bills for consideration. Bill 1 promotes the use of hand sanitizer before every handshake and every human interaction; it would be illegal for somebody to not use hand sanitizer before interacting with another person. Bill 2 promotes the use of condoms; it would be illegal under Bill 2 to engage in sexual intercourse without a condom, except at specially designated times reserved for procreation. Suppose that both bills would be accompanied by a massive public information campaign and incentive program, e.g free condoms and/hand santitizer. Explore the effects of these bills, assuming that they changed behavior, on the trajectory of pathogen evolution. Consider either directly transmitted or sexually transmitted diseases (or both). How would these bills change virulence, the duration of infectivity, transmission rates, and mortality of these illnesses? Would these bills have a chance of really altering pathogen behavior if they were enacted into law, knowing something about human nature?

Good Luck!

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Joe Alcock

Emergency Physician, Educator, Researcher, interested in the microbiome, evolution, and medicine