Researchers in the United States believe they have discovered why flu and other respiratory viruses are more prevalent in winter.
The researchers from the National Institute of Child Health and Human Development (NICHD) say influenza viruses coat themselves in a fatty protein called hemagglutinin, that hardens and protects them in colder temperatures.
They suggest that this may be the reason why winter is the flu season.
It seems the fatty coating around the virus melts in the respiratory tract, allowing the virus to infect cells and it is this special insulator which gives them an advantage in the cold temperatures of the winter.
When the influenza viruses are in a warm environment such in summer, they die quickly and are usually less able to infect more than one person.
Lead researcher Joshua Zimmerberg says it is only in this liquid phase that the virus is capable of entering a cell to infect it.
Experts have long debated why it is that flu and other respiratory are more common in winter and come up with theories such as cold weather forcing people indoors in possibly confined situations or the destructive effect of the sun's radiation in summer.
NICHD Director Duane Alexander says the findings could lead to new ways to prevent and treat flu by exploring ways to interfere with the protective mechanism and could also help scientists find new ways to eradicate influenza.
The researchers used nuclear magnetic resonance imaging to look at the outer coat of flu viruses.
The researchers say as viruses cannot replicate on their own they hijack a living cell by fusing the outer coating to the victim cell, then by injecting genetic material into the cell they create a virus factory.
In some cases certain viruses then simply explode out of these hijacked cells, whereas influenza viruses "bud" out using lipids such as cholesterol from the cells to help make the membrane.
While inside the warm cell, the hemagglutinin is liquid but at cooler temperatures it starts a process called ordering and gradually solidifies from 40 degrees C down to 4 degrees C.
Outside in warmer temperatures this protective coating melts and unless it is inside a living person or animal, the virus perishes.
In cold temperatures, the hard lipid shell might even be able to withstand certain detergents, making it more difficult to wash the virus off hands and surfaces.
Influenza and other respiratory viruses are spread in small droplets by coughing, sneezing and talking and they can also settle onto surfaces, to be picked up on fingertips.
The new research is published in the journal Nature Chemical Biology.