An article in Slate last month catalogs the reasons why longevity has remarkably increased over the last 150 years.
Laura Helmuth writes in that article that:
“Some credit for the historical decrease in deadly diseases may go to the disease agents themselves. The microbes that cause rheumatic fever, scarlet fever, and a few other diseases may have evolved to become less deadly. Evolutionarily, that makes sense—it’s no advantage to a parasite to kill its own host, and less-deadly strains may have spread more readily in the human population. Of course, sudden evolutionary change in microbes can go the other way, too: The pandemic influenza of 1918–19 was a new strain that killed more people than any disease outbreak in history—around 50 million. In any battle between microbes and mammals, the smart money is on the microbes.”
We are going to discuss the central evolutionary idea expressed in that paragraph in class, namely, that parasites will evolve towards a more benign state over time because it is not in the parasite’s interest to kill its own host. Clearly, though, parasites do kill a lot of people, every day in fact. When do you suppose it might be in the parasites interest to kill its host? Is such a thing possible?