Gupta has watched it happen. As described in a Nature paper last year, he and his colleagues observed a patient suffering from persistent COVID-19. The patient was a man in his 70s who had previously suffered from lymphoma. Over the course of 101 days, the team took viral samples 23 times. They found that the virus was continually undergoing mutations, some of which helped the virus evade the host’s immune defenses. As time went by, the different substrains within the patient continuously jostled with one another for dominance. Gupta’s team was witnessing evolution in real time.

Infections like this, Gupta says, are the nurseries of dangerous new variants. And given the scale of the pandemic, nature is churning them out far more effectively than any human lab could. Last year, there was an extended debate in the media over speculation that the original SARS-CoV-2 virus had been created in a facility in Wuhan, China, perhaps as a result of “gain of function” research, in which scientists apply artificial selection to lab-bred strains to try to generate a more transmissible version of an existing virus. Whether or not this actually happened, a similar process goes on naturally inside of the body of every COVID-19 patient suffering a prolonged infection. By the virus’s natural function, it is effectively turning its host into an efficient gain-of-function lab.