Scientists have found a link between a heightened risk of flu and rapid weather swings that have become increasingly common in recent years due to climate change. The study, published in the journal Environmental Research Letters, examined more than a decade of health and weather data in the United States, mainland China, Italy, and France.

Historically, low temperatures and humidity in the winter were thought to foster the optimum conditions for transmitting the flu virus. But the 2017-2018 flu season was one of the warmest on record, and was also one of the deadliest, with 186 children dying in the U.S., according to the Centers for Disease Control. The fall of 2017 saw wild swings in weather, and scientists at Florida State University and Nanjing University in China were curious whether those fluctuations were to blame for the severe flu season.

The researchers analyzed surface temperatures, weather patterns, and health records from January 1, 1997 to February 28, 2018, a total of 7,729 days. They found that years with intense weather fluctuations in the autumn months incited the flu, creating a robust patient population early in flu season that continued to grow throughout the winter.

“The historical flu data from different parts of the world showed that the spread of flu epidemic has been more closely tied to rapid weather variability, implying that the lapsed human immune system in winter caused by rapidly changing weather makes a person more susceptible to flu virus,” Zhaohua Wu, an atmospheric scientist at Florida State University and coauthor of the new study, said in a statement.

The international team of researchers said that this trend would only continue to get worse as climate change drives more extreme and variable weather conditions. According to climate models, “rapid weather variability in autumn will continue to strengthen in some regions of northern mid-latitudes in a warming climate, implying that the risk of influenza epidemic may increase 20 percent to 50 percent in some highly populated regions in later 21st century,” the study concluded.

Wu and his colleagues said their findings should be included in future flu spread models. “The autumn rapid weather variability and its characteristic change in a warming climate may serve not only as a skillful predictor for spread of flu in the following season but also a good estimator of future flu risk,” he said.

For more on how climate change is driving wild swings in weather, click here.