The unusual cases of pneumonia began to appear in midwinter, in China. The cause, researchers would later learn, was a coronavirus new to science. By March, the infection began to spread to other Asian countries and overseas. People were dying, and the World Health Organization issued a global health alert. But this was 2003, not 2020, and the disease was SARS, not COVID-19. By June, the outbreak was almost gone, with just 8,098 confirmed infections and 774 deaths worldwide. No cases of SARS have been reported since 2004. Contrast that with the closely related coronavirus that causes COVID-19 today: More than 13,600,000 confirmed cases as of July 16, and more than 585,000 deaths.
Why did SARS go away while today’s coronavirus just keeps on spreading? Why, for that matter, did both these coronaviruses spill over into people at all, from their original bat hosts?
And just as vital as those questions is another: What happens next?
As we face the current pandemic, it will be important to understand how SARS-CoV-2, the virus that causes COVID-19, is likely to evolve in the months and years ahead. It’s possible the virus could lose its lethal character and settle into an evolutionary détente with humanity. It might end up as just another cold virus, as may have happened to another coronavirus in the past. But it could also remain a serious threat or perhaps even evolve to become more lethal. The outcome depends on the complex and sometimes subtle interplay of ecological and evolutionary forces that shape how viruses and their hosts respond to one another.
“One thing you learn about evolution is never to generalize,” says Edward Holmes, an evolutionary virologist at the University of Sydney, Australia, and author of an article on the evolution of emerging viruses in the Annual Review of Ecology, Evolution, and Systematics. “It depends entirely on the biological nuance of the situation.”
