6 ideas
A pathogen's virulence evolves in response to how it spreads: when transmission depends on a mobile, healthy host, natural selection favors mild strains, but when transmission can occur from an immobilized or dead host, selection favors aggressive exploitation. The host's required mobility for spreading acts as a brake on how much damage the pathogen can profitably inflict.
Pathogens that survive long periods in the external environment outside a host can afford to be highly virulent, because they do not need their current host to remain alive or mobile to reach the next one. Environmental durability decouples a pathogen's reproductive success from its host's survival, removing the evolutionary penalty for killing the host.
Diseases transmitted by mosquitoes or other vectors tend toward high virulence because an incapacitated host is still accessible to a biting insect, severing the link between host mobility and pathogen spread. The vector does the traveling, so the pathogen pays no cost for debilitating its host.
Public health interventions can be designed not just to block transmission but to alter the selective pressures so that benign strains outcompete virulent ones. Clean water supplies, mosquito-proofed housing, and attendant hygiene can convert deadly pathogens into mild ones by removing the routes that reward virulence.
Many conditions long attributed to genetics or lifestyle—certain cancers, heart disease, mental illnesses—may actually be caused by infectious agents acting over long timescales. The apparent absence of an obvious microbe reflects the limits of detection and the slow, indirect ways pathogens can damage tissue, not the true absence of a pathogen.
Vibrio cholerae strains became more lethal where waterborne transmission let the pathogen spread even from bedridden victims, and milder where clean water forced reliance on person-to-person contact requiring a functional host. Comparing regions with differing water sanitation showed virulence tracking the transmission route rather than the bacterium's intrinsic nature.