No extinction from disease – natural immunization and modern medicine solve, only 1 mammal has ever been wiped out by disease

THE PERFECT STORM Will these inequities eventually lead to a species-wide downfall? It's not easy to calculate the likelihood that, say, the climate emergency will kill us all, says Luke Kemp, a research affiliate at the Center for the Study of Existential Risk at the University of Cambridge. But it's probably not realistic to consider risks individually anyway, he says. "When we look at the history of things like mass extinctions and societal collapses, it's never just one thing that happens," Kemp says. "If you're trying to rely on a single silver bullet to kill everyone in a single event, you have to write sci-fi." The end of humanity is far more likely to be brought about by multiple factors, Kemp says—a pileup of disasters. Although apocalyptic movies often turn to viruses, bacteria and fungi to wipe out huge swathes of the population, a pandemic alone probably won't drive humanity to extinction simply because the immune system is a broad and effective defense, says Amesh Adalja, an infectious disease physician and senior scholar at the Johns Hopkins Center for Health Security. A pandemic could be devastating and lead to severe upheaval—the Black Death killed 30 to 50 percent of Europe's population—but it's unlikely that a pathogen would kill all of humanity, Adalja says. "Yes, an infectious disease could kill a lot of people," he says, "but then you're going to have a group [of people] that are resilient to it and survive." Humans also have tools to fight back against a pathogen, from medical treatments to vaccines to the social-distancing measures that became familiar worldwide during the COVID pandemic, Adalja says. There is one example of a mammalian species that may have been entirely eliminated by an infectious disease, he says: the Christmas Island rat (Rattus macleari), also called Maclear's rat, an endemic island species that may have gone extinct because of the introduction of a parasite. "We are not helpless like the Christmas Island rat, which couldn't get away from that island," Adalja says. "We have the ability to change our fate."

If infectious disease contributes to the downfall of humanity, it will probably be as just one piece of a larger puzzle. Imagine a planet pushed to upheaval by sea-level rise and disruption to agriculture from climate change. The humans of this climate-ravaged world attempt a geoengineering solution that goes wrong. The situation worsens. Resources are scarce, and a bunch of countries have nuclear weapons. Oh, by the way, the mosquitoes that carry yellow fever range as far north as Canada in this scenario. It's not hard to see how the human population could decline and disappear in the face of an arsenal of challenges, according to Kemp. Worst-case scenarios are understudied, Kemp says. In climate science, for example, there is a lot of research into what the world might look like at two or three degrees Celsius warmer than the preindustrial average but very little looking at what an increase of five or six degrees C might look like. This is partly because scientists have a hard time predicting the effects of that much warming and partly because climate scientists feel pressure from politicians not to appear alarmist, Kemp says. Models of future worst-case scenarios also tend to do an inadequate job of predicting the cascading effects of a disaster. "The general field of existential risk is relatively new, nascent and just understudied," Kemp says. There are questions as to how much humans should worry about something as big picture as extinction. While some see the question as pressing—controversial tech billionaires such as Elon Musk and Peter Thiel have funded organizations dedicated to studying the risks of transformative technologies—others argue that today's problems are urgent enough. Already humans are heating the globe, overexploiting and destroying nature, using land and water unsustainably, and creating chemicals that are harmful to all life, often in service to the globally well-off, says Sarah Cornell, who studies global sustainability at the Stockholm Resilience Center at Stockholm University. "Today's reality is that some human beings are undermining or even destroying living conditions of many, many other people," Cornell says. "From a human-scale perspective, this is an existential crisis already, not a risk somewhere up ahead."