A finding by a team of scientists at the National Institutes of Health may account for why the flu virus is more infectious in cold winter temperatures than during the warmer months.

At winter temperatures, the virus's outer covering, or envelope, hardens to a rubbery gel that could shield the virus as it passes from person to person, the researchers have found. At warmer temperatures, however, the protective gel melts to a liquid phase. But this liquid phase apparently isn't tough enough to protect the virus against the elements, and so the virus loses its ability to spread from person to person.

The full study is here:

Using linewidth and spinning sideband intensities of lipid hydrocarbon chain resonances in proton magic angle spinning NMR spectra, we detected the temperature-dependent phase state of naturally occurring lipids of intact influenza virus without exogenous probes. Increasingly, below 41 °C ordered and disordered lipid domains coexisted for the viral envelope and extracts thereof. At 22 °C much lipid was in a gel phase, the fraction of which reversibly increased with cholesterol depletion. Diffusion measurements and fluorescence microscopy independently confirmed the existence of gel-phase domains. Thus the existence of ordered regions of lipids in biological membranes is now demonstrated. Above the physiological temperatures of influenza infection, the physical properties of viral envelope lipids, regardless of protein content, were indistinguishable from those of the disordered fraction. Viral fusion appears to be uncorrelated to ordered lipid content. Lipid ordering may contribute to viral stability at lower temperatures, which has recently been found to be critical for airborne transmission.

NIH Scientists Offer Explanation for Winter Flu Season