Why The Flu Sets In During Winter?

This has been a common question among both clinicians and lay folks. Recent studies seem to indicate that high absolute humidity in the atmosphere lowers the survivability of the virus.
From a New York Times interview with Dr. Jeffrey Shaman, an atmospheric scientist at Oregon State University, a lead author of the latest study cited below:

Can you describe your results?
We re-examined the data from those laboratory investigations of influenza virus survival and transmission and looked for a connection with absolute humidity. During our investigation, we found that both the survival and transmission of influenza virus increase markedly in lower absolute humidity conditions. While relative humidity only explains a small amount of this variability of influenza virus survival and transmission (36 percent and 12 percent, respectively), absolute humidity accounts for much more (90 percent and 50 percent, respectively).
Additionally, in temperate regions both outdoor and indoor absolute humidity are at a minimum during winter months, conditions that favor influenza survival and transmission. The same is not true for relative humidity, which is at a maximum outdoors during winter. These findings indicate that absolute humidity provides a single, coherent and more physically sound explanation for observed changes in the survival, transmission and seasonality of influenza.
Should people raise the humidity in their homes in the wintertime to stave off the flu?
The best line of defense against influenza remains vaccination. Our results do suggest that during an influenza outbreak, it may be worthwhile to humidify the home and workplace. However, there are many pathogens, such as mold, that proliferate in higher humidity conditions. There is a trade-off, so I would not recommend increasing humidity without consideration of the effect on these other potentially harmful agents.

More from the New York Times…
Abstract in PNAS…
Flashback: Study Explains Winter Flu Season
Image: Influenza viruses (purple particles around the upper surface of the endosome) in an infected cell. These viruses are in the process of being uncoated before replicting in the host cell. The endosome is acidic which triggers the haemagglutinin on the surface of the virus to begin the uncoating process. Wellcome Images