Story highlights The first really cold week of the season triggered flu outbreak, a Swedish study found

Airborne viruses can spread more easily in cold, dry air, researchers say

Cases of flu are on the rise, according to a recent statement from the United States Centers for Disease Control and Prevention, and experts are warning that this year's flu season will be worse than last.

Now, a new study published in the Journal of Clinical Virology is shedding some light on exactly how cold weather and the spread of viruses are linked.

It turns out, seasonal flu outbreaks first appear each year about a week after the winter's first cold spell -- or at least that's what happened in Sweden, over the course of three years when researchers tracked weather patterns and the prevalence of the virus.

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

A surprisingly consistent pattern emerged: Each year, the first really cold week -- with low humidity and temperatures below freezing -- seemed to trigger the spread of flu.

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