Flu is spreading across the country, picking up speed as the temperature drops. But why do we have flu epidemics every winter instead of every summer?

Using a combination of 20,000 virus samples and weather statistics compiled over three seasons, researchers at Sweden's Sahlgrenska Academy think they have the answer: Flu outbreaks usually begin one week following the first really cold period with low outdoor temperatures and low humidity.

"According to our calculations, a cold week with an average temperature below zero degree Celsius precedes the start of the flu epidemic" says Nicklas Sundell, a researcher at Sahlgrenska Academy and infectious diseases specialist at Sahlgrenska University Hospital. Zero degree Celsius is equal to 32 degrees Fahrenheit.

"We believe that this sudden drop in temperature contributes to 'kickstart' the epidemic," says Sundell. "Once the epidemic has started, it continues even if temperatures rise. Once people are sick and contagious, many more may become infected."

The study supports the theory that aerosol particles containing virus and liquid spread more readily in cold and dry weather. If the air is dry, it absorbs moisture and the aerosol particles shrink and can remain airborne.

"Cold and dry weather and small aerosol particles are important perquisites for the flu epidemic to take off," he said. "But cold weather isn't the only contributing factor. The virus has to be present among the population and there have to be enough people susceptible to the infection."

Cold, dry weather also seems to aid the spread of other common viruses that cause respiratory infections, such as the RS-virus and coronarvirus. Conversely, some viruses such as rhinovirus, a common cause of colds, aren't affected by the weather and are present year-round.

"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 and prepare emergency wards and hospital staff in advance for an increased number of patients seeking care," Sundell said.

The Centers for Disease Control and Prevention announced last week that this season may be severe due to the fact that the prominent strain is H3N2, which often indicates a severe season.

Each year, about 200,000 Americans are hospitalized due to flu complications, including pneumonia and more than 36,000 people die.