That conclusion is based on the first experiments investigating how natural selection influences the transmission of infectious disease. The outcome of those experiments defies old assumptions that pathogens evolve to become less infectious
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The scientists analyzed the patterns of disease transmission and found that strains of bacteria with the greatest damage to their virulence genes were slowest at spreading from one host to another. The strain of pathogen with all of its virulence genes intact spread the fastest. Strains of pathogen lacking the ability to inject any proteins into the host were completely unable to spread between hosts, suggesting that these swapped virulence genes are essential for spread.