Charles Darwin himself most likely could not have imagined the impact of his ideas on the way today's scientists interpret the spread and power of infectious diseases.
Scientists are using evolutionary concepts to explain why some viruses and bacteria are highly virulent and life-threatening, while others reside in their hosts with few, if any, ill effects. By formulating mathematical models from evolutionary forces at play, scientists are beginning to predict patterns of disease transmission and their levels of virulence.
Understanding these concepts, one has to look at infections from the point of view of the microbes : They're just another population of organisms trying to make a living by exploiting their environment (usually an infected host).
A microbe or parasite must balance the amount of harm it does to its unwilling host against its ability to transmit itself. The more easily the bugs can spread, the less it matters whether they seriously sicken and kill their hosts. That's the case with the bug that causes cholera . As a diarrheal parasite, the bacterium spreads easily through bed sheets, clothes, and sewage-contaminated water. Therefore, virulence remains high, and infected people suffer and die.
When water is purified and kept clean, the bugs cannot spread as rapidly. Thus the bacteria that can spread are the milder form that do not kill their host. In India in the 1950s and 1960s, a campaign to clean up water led to a milder form of cholera displacing the more dangerous classic type.
Another example relates to HIV . This virus is more virulent when sexual activity is frequent and involves many partners, favoring HIV that reproduces rapidly.
When there is less sexual activity, the virus has fewer chances for transmitting its genes to another host. Under these conditions, natural selection favors forms of the virus that are more latent, allowing their host to stay healthy and active. Some scientists suggest that HIV might become less virulent over time, either because it infects so much of the population that there are no further opportunities for spread, or because shifts in behavior patterns cut down opportunities for infection.
Seen through the evolutionary lens, it's understandable why certain diseases are so virulent and deadly. Malaria, yellow fever, typhus, and sleeping sickness are spread by biting insects like mosquitoes. Their hosts do not have to survive very long, because mosquitoes transmit the pathogens , so there is no selection pressure for the diseases to be mild.
Putting these insights into practice may make it possible to tame the potency of such devastating diseases. By taking public health measures that slow the transmission of diseases, we influence the selection of less virulent bugs.