In the Northern hemisphere, winter is the time for the flu. Every year 5% to 20% of us catch "the bug". So predictable is the influenza virus that "flu season" has entered the vernacular. This year, flu cases peaked around the end of February (see chart). Perhaps you've wondered "Why?".

Hypotheses for flu season are numerous and include:

1. Because people are indoors more often during the winter, they are in close contact more often, and this promotes transmission from person to person.
2. Cold temperatures lead to drier air, which may dehydrate mucus, preventing the body from effectively expelling virus particles.
3. The virus may linger longer on exposed surfaces (doorknobs, countertops, etc.) in colder temperatures.
4. Increased travel and visitation due to the holiday season.
5. Less sunlight promotes virus survival.
6. Our immune systems work poorly during the cold weather. (From Wikipedia).

Researchers from the National Institutes of Health have added another hypothesis to this list: influenza viruses get harder during cold temperatures.

See, influenza viruses have a lipid coat. Most of us are familiar with lipids in the form of oils we use for cooking. Whether the lipid is solid or liquid depends on the temperature. Olive oil, for example, is liquid at room temperature, but will solidify if you place it in the freezer.

So too for the influenza virus. Its lipid coat helps protect it from the elements, but is only good when it is tough and rubbery. In a study reported in Nature Chemical Biology, NIH researchers used a sophisticated magnetic resonance technique, developed and previously tested in NIAAA's Laboratory of Membrane Biochemistry and Biophysics, to create a detailed fingerprint of how the virus’s outer membrane responded to variations in temperature. At low temperatures, the lipid coat was solidified as a gel. As the temperature approached 60 degrees Fahrenheit, the coat turned into a goopy mess.

We spread the flu from person to person when we cough and sneeze. In cold temperatures, the virus is better able to survive the elements and find a new host. Once the virus enters a host, the outer protective coat "melts like an M&M in your mouth", and enables the virus to enter the host's cells.

Dr. Joshua Zimmerberg, corresponding author of the study, suggested that people might better protect themselves against getting sick by remaining indoors at warmer temperatures than usual.

Progressive ordering with decreasing temperature of the phospholipids of influenza virus. Ivan V Polozov, Ludmila Bezrukov, Klaus Gawrisch, Joshua Zimmerberg. Nature Chemical Biology 4, 248 - 255 (01 Apr 2008), doi: 10.1038/nchembio.77, Article

Chart from the Centers for Disease Control.

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