Why Hasn't Disease Wiped out the Human Race?

“You’ll tell us when you’re worried, right?”

That was the question posed to me countless times at the height of the 2014 West African Ebola outbreak. As an infectious disease physician, I was interviewed on outlets such as CNN, NPR, and Fox News about the dangers of the virus, and the answer I gave was always the same: “Ebola is a deadly, scary disease, but it is not that contagious. It will not find the U.S. or other industrialized nations hospitable.”

In other words, no, I wasn’t worried—and not because I have a rosy outlook on infectious diseases. I’m well-aware of the damage these diseases are causing around the world: HIV, malaria, tuberculosis; the influenza pandemic that took the world by surprise in 2009; the anti-vaccine movement bumping cases of measles to an all-time post-vaccine-era high; antibiotic-resistant bacteria threatening to collapse the entire structure of modern medicine—all these, like Ebola, are continuously placing an enormous number of lives at risk.

But when people ask me if I’m worried about infectious diseases, they’re often not asking about the threat to human lives; they’re asking about the threat to human life. With each outbreak of a headline-grabbing emerging infectious disease comes a fear of extinction itself. The fear envisions a large proportion of humans succumbing to infection, leaving no survivors or so few that the species can’t be sustained.