Wendy Orent in Undark:
As far as scientists and historians can tell, the bacterium that caused the Black Death never lost its virulence, or deadliness. But the pathogen responsible for the 1918 influenza pandemic, which still wanders the planet as a strain of seasonal flu, evolved to become less deadly, and it’s possible that the pathogen for the 2009 H1N1 pandemic did the same. Will SARS-CoV-2, the virus that causes Covid-19, follow a similar trajectory? Some scientists say the virus has already evolved in a way that makes it easier to transmit. But as for a possible decline in virulence, most everyone says it’s too soon to tell. Looking to the past, however, may offer some clues.
The idea that circulating pathogens gradually become less deadly over time is very old. It seems to have originated in the writings of a 19th-century physician, Theobald Smith, who first suggested that there is a “delicate equilibrium” between parasite and host, and argued that, over time, the deadliness of a pathogen should decline since it is really not in the interest of a germ to kill its host. This notion became conventional wisdom for many years, but by the 1980s, researchers had begun challenging the idea.