To model what might have been happening during the swine flu pandemic, the researchers grew human airway tissue in the lab and infected it with rhinovirus. Then, three days later, they gave it the H1N1 flu. They were intrigued to see that the flu virus just fizzled out, and they determined that the rhinovirus had switched on a number of genes that produce innate immune proteins. Suspecting that molecular messengers called interferons had flipped those switches, they treated the tissues with a drug that blocked interferons and ran the experiment again. “Lo and behold, the influenza grows just fine,” says Foxman. Interferons produced to fight the rhinovirus had been beating back the flu.

A number of viruses trigger the interferon response, and it’s possible that any of them could make the body put up stiff resistance to a new infection for some period of time. For instance, the team didn’t test whether having the flu first would stop a rhinovirus in its tracks, but it’s plausible, says Foxman. That might explain why flus and colds have alternating peaks every year. There are a lot of reasons why one virus might take center stage over others, including human behavior, school schedules, and climate. “But you really wonder if viral interference is one missing piece of that equation,” Foxman says.

In the current pandemic, the same questions are at play. While social distancing and masks are reducing the incidence of seasonal flu, perhaps the prevalence of Covid-19 is cutting it down further. Or, says Schultz-Cherry, maybe the flu would have slowed down Covid-19. They’re questions that can only be answered with further research, but they are worth asking.