Geography and history

Yet despite epidemic after epidemic, despite mass killers like smallpox and the 1918 flu, at no point has disease threatened humans with extinction. Even the Black Death, likely the most concentrated epidemic of all time, now appears as little more than a minor downturn in what has otherwise been a bull market for long-term human population growth. That’s true for animals as well. The International Union for Conservation of Nature reports that of the 833 plant and animal extinctions that have been documented since 1500, less than 4 percent can be attributed to infectious disease. Those species that were eradicated by disease tended to be small in number and geographically isolated—very much unlike human beings, who are both numerous and have spread to every corner of the world.38

With the exception of HIV—which can now be managed as a chronic condition with antiviral drugs—every major epidemic mentioned above took place before the dawn of modern medicine, before the development of antibiotics and widespread vaccines. Smallpox was even fully eradicated from the wild in 198039—the only known samples of the virus are kept at highly secure government facilities in Atlanta and Koltsovo, Russia.40 Plague is now so rare that when it breaks out in countries like Madagascar, it makes global news—yet fewer than 600 deaths from the disease were reported between 2010 and 2015. Studies have shown that most of the fatalities from the 1918 flu were actually due to secondary bacterial infections that today could be controlled by antibiotics,41 which were introduced less than a century ago. Influenza pandemics remain the great fear of infectious disease experts, but the most recent one in 2009 killed only about 284,000 people worldwide.42 That was fewer than the number of people who die from seasonal flu in an ordinary year.43

Modern science has defanged most infectious diseases, at least outside the developing world—and great progress has been made there in recent years—but basic evolution also plays a role in limiting the catastrophic potential of natural disease. Every pathogen faces a trade-off. In general, the more rapidly it kills, the harder it is to spread widely, because an extremely virulent disease would run out of victims and hit an epidemiological dead end. Pathogens that are highly transmissible, like influenza, rarely kill, even absent the countermeasures of modern medicine. The 1918 flu had a fatality rate of about 2.5 percent.44 That’s tremendously high by the standards of the flu, but it still meant that more than 97 out of every 100 patients survived. Even a virus like HIV—which kills slowly and shows no symptoms for years, permitting the infected plenty of time to spread the disease—is hindered because transmission requires direct contact with blood or with bodily fluids. The self-replication that makes infectious disease such an effective weapon also prevents it from becoming a true existential threat. What viruses and bacteria want—if packets of genes and single-celled organisms can be said to want anything—is to survive and to replicate. They can’t do that if they kill all humans.