Flu season has long been associated with winter, but a new study suggests transmission of the disease has more to do with humidity than with temperature. The heart of the study is this: Researchers don’t really know why flu is seasonal, said Univesity of Arizona's Andrew Comrie. So he and his team, including scientists from the National Institutes of Health, examined a global database of 78 locations that showed peaks in flu outbreaks going back to 1975.

In places like North America, influenza was more common just after periods of very low humidity. In tropical locations, the team found just the opposite. Flu was most widespread during the rainy season.

"When it’s very humid. So [it’s] the two extremes. Flu is not high in the middle – in the normal set of range of humidity," Comrie explained.

These findings could help target flu surveillance efforts, and improve the timing of seasonal vaccines, the study's authors said.

The paper was published this week in the journal PLoS Pathogens.