You know the drill: when the winter coat comes out, so do the pocket packs of tissues.* Cold weather and flu season are pretty much synonymous for most of us. Yet there are plenty of areas in the world that never get cold—and the flu still finds them anyway. Now researchers say changes in humidity could help explain why tropical regions still experience outbreaks of seasonal flu. They published their findings in the Proceedings of the National Academy of Sciences.

The flu virus (or viruses, really) is an unfussy traveler and can make itself at home in a number of different climates, but the forces underlying its seasonal cycles have been little understood. Previous studies have shown that both relative and absolute humidity can affect the rate at which droplets travel through the air and thus how fast the flu spreads, while others found that mammals tend to spread the virus faster in cold climates. But all of these studies were performed in laboratories, using guinea pigs and machines. No one could say whether their results would translate to the germ-filled real world.

Figuring that out would require a broad range in expertise, including climate science, epidemiology, preventive medicine, and bioengineering. So researchers at three California institutions formed a sort of interdisciplinary super team, which would allow them to combine both their know-how and their relevant data.

The team decided to use a technique called empirical dynamic modeling, or EDM, which is pretty much exactly what it sounds like: It combines real-world data with mathematical modeling to study complex, constantly fluctuating systems like our global climate or the ebb and flow of an ecosystem.

Their first dataset came from the World Health Organization’s Global Health Atlas: all worldwide records of laboratory-confirmed influenza A or B diagnoses from 1996 to 2014. Next they turned to the National Oceanic and Atmospheric Administration’s Global Surface Summary of the Day, which provided week-by-week records of temperature and absolute humidity for the same time period.

By feeding these data into an EDM representation of the planet, the team was able to get a zoomed-out perspective of the interplay between weather and the spread of disease. They found that it was not temperature that drove flu outbreaks, nor humidity—it was the combination of the two. In cold climates, the virus prefers low humidity and dry weather. But when temperatures rise, the flu luxuriates in damp, humid conditions like those in the tropics.

"The analysis allowed us to see what environmental factors were driving influenza," Scripps Institution of Oceanography's George Sugihara, a co-author of the study, said in a press statement. "We found that it wasn't one factor by itself, but temperature and humidity together."

These findings could have real implications in the global fight against the flu, the authors write. They suggest that setting up humidifiers in cold, dry places and dehumidifiers in the tropics could create environments so unfriendly to viruses that even the flu can’t stick around.

*Please consider this your reminder to get your flu shot.