COVID was a “very bad pandemic,” with more than 1.1 million deaths in the U.S. alone so far, Dr. Robert Wachter, chair of the Department of Medicine at the University of California, San Francisco, noted. Wachter—the author of 300 articles and six books, including a New York Times bestseller—became popular on Twitter during the pandemic.

COVID wasn’t the beast it could have been in terms of mortality, he argued. The virus generally killed under 5% of the people it infected, and, depending on which country you resided in, often much less. Fellow coronaviruses SARS and MERS, on the other hand, were much less transmissible but more deadly, with case fatality rates around 10% and 34%, respectively.

When it comes to the next pandemic, Wachter said he is “quite worried,” and that it could be worse than COVID.

“There’s nothing in the book of life that says you couldn’t have a virus that’s as infectious as this one but far more lethal,” he noted.

COVID as an evolving threat

Wachter isn’t the only expert to raise the possibility of an equally transmissible but more lethal pandemic pathogen. COVID’s ability to infect more efficiently has skyrocketed since 2019, soaring from near the bottom of the list of contagious diseases to near the top, where it battles with measles for supremacy.

It’s possible that ultra-transmissible Omicron evolves to become more deadly, experts warn—though there’s no telling just how likely this scenario is, or when the transition might occur, if it ever does. That said, such a development may not be far off. Scientists are watching COVID evolution for the potential development of a variant that has Omicron’s transmissibility with the lethality of Delta.

Such a scenario, while not a “nightmare,” would be “a problem,” Raj Rajnarayanan, assistant dean of research and associate professor at the New York Institute of Technology campus in Jonesboro, Ark., and a top COVID-variant tracker, recently told Fortune.

“What’s to say that we’re not going to eventually see a COVID that has both?” Dr. Michael Osterholm, director of the University of Minnesota’s Center for Infectious Disease Research and Policy (CIDRAP), told Fortune last fall. He was speaking of transmissibility and the lethality of SARS, MERS, or worse.

If COVID evolution were to take a turn for the worse, powers that be would need to decide whether it constituted a new pandemic and warranted a new name entirely—perhaps SARS-CoV-3—or if it was simply an extension of the current pandemic, which is still ongoing, according to the World Health Organization.

Since Omicron burst onto the scene nearly a year and a half ago, evolution, while speedy, hasn’t resulted in any major changes in how the virus presents, though each new major variant tends to chip away a little more at immunity and/or become a bit more transmissible, Wachter said. But “a new variant could come out tomorrow. It could laugh at your prior immunity.”

The chances of such a scenario are likely less than 20%, Wachter and colleagues estimate. The largely survivable but less-than-harmonious coexistence with the virus we’re experiencing today will likely be unchanged three years from now, he added.

Other viral threats are possible

The next pandemic pathogen may not be a coronavirus at all. Experts are eyeing a variety of strains of bird flu, given increasing transmission to and among mammals, and several recent human cases in disparate parts of the world.

There’s always the possibility of something entirely new. Among the list of the WHO’s “priority pathogens” that have the potential to cause outbreaks and pandemics is “Disease X,” which represents an unknown threat.

That list also includes Crimean-Congo hemorrhagic fever; the Ebola and Marburg viruses; Lassa fever; coronaviruses SARS, MERS, and COVID-19; henipaviruses, and Rift Valley fever. The list was last updated in 2019, and a revised list should be released this year, according to the WHO.