This butter-like coating melts in the respiratory tract, allowing the virus to infect cells, the team at the National Institutes of Health found.

"Like an M&M in your mouth, the protective covering melts when it enters the respiratory tract," said Joshua Zimmerberg of the National Institute of Child Health and Human Development (NICHD), who led the study.

"It's only in this liquid phase that the virus is capable of entering a cell to infect it."

Experts have long pondered why flu and other respiratory viruses spread more in winter. No one explanation, such as people staying indoors more, or the destructive effect of the sun's radiation in summer, has fully explained it.

The new report, published in the journal Nature Chemical Biology, could lead to new ways to prevent and treat flu, said NICHD Director Duane Alexander.

"The study results open new avenues of research for thwarting winter flu outbreaks," Alexander said in a statement.

"Now that we understand how the flu virus protects itself so that it can spread from person to person, we can work on ways to interfere with that protective mechanism."

Zimmerman's team used a type of imaging called nuclear magnetic resonance imaging to look at the outer coat of flu viruses.