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And what you can do to avoid getting sick.

It’s flu season, and the dreaded virus is back and badder than before. The U.S. Centers for Disease Control and Prevention are warning that this year’s flu season will be worse than last.

So what is it about cold weather that causes the flu to thrive? A new study published in the Journal of Clinical Virology has some answers. After tracking weather patterns and the prevalence of the virus in Sweden for three years, researchers found that seasonal flu outbreaks tend to appear each year about a week after the winter’s first cold spell.

During that time, researchers collected more than 20,000 nasal swabs from people seeking medical care in and around the city of Gothenburg. Then, they analyzed them for influenza and other respiratory viruses. Next, they compared those findings with weather data from the Swedish Meteorological and Hydrological Institute.

Eventually, a pattern began to take shape. Each year, the first really cold week—with low humidity and temperatures below freezing—seemed to trigger the spread of flu.

“We believe that this sudden drop in temperature contributes to ‘kickstart’ the epidemic,” said lead author Nicklas Sundell, a researcher at Sahlgrenska Academy and infectious diseases specialist at Sahlgrenska University Hospital, in a press release. “Once the epidemic has started, it continues even if temperatures rise. Once people are sick and contagious, many more may become infected.”

Furthermore, airborne particles containing liquid and the flu virus—like a sneeze or cough—can spread more easily in cold and dry weather. Dry air absorbs moisture from the particles, which causes them to shrink, helping them stay in the air longer and travel greater distances.

The hope is, Sundell says, that better knowledge of outbreaks based on weather may help experts to know what’s coming.

“If you can predict the start of the annual epidemics of the flu and other respiratory viruses, you can use this knowledge to promote campaigns for the flu vaccine,” he said.

For now, Sundell recommends covering your face when you cough and sneeze, washing your hands frequently, and, of course, getting a flu shot.

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