The first time a severe infectious disease tears through a new population, it’s sure to wreak havoc. Without previous exposure, no members of the community are immune, leaving the virus with numerous potential hosts to sustain it for months to come, regardless of the weather forecast.

Columbia University epidemiologist Micaela Martinez compares early outbreaks to a fire igniting in a forest full of kindling. The occasional rainstorm might do a bit to slow the conflagration. But with so many vulnerable trees, a touch of precipitation would be nowhere near enough to snuff out the flames. “For the first wave, the seasonality is not as relevant,” she says. “We can’t expect [the virus] to just go away.”

Once the current pandemic subsides, however, future infections would propagate amongst a population with a smaller proportion of immune individuals. These likely tamer outbreaks could reveal a seasonal cycle, which Martinez believes is a quality ubiquitous among infectious diseases. In 2018, she set out to catalog these trends and was surprised to find that all of the nearly 70 infections she studied showed some sort of seasonal rise and fall.