Cold and dry. That's Denver in winter as opposed to the summer when it is hot and dry. Now New York virologists have an explanation as to why influenza virus plagues us here--and throughout the US--in the winter and not in the summer.

No new news that flu flogs us in winter. Epidemiologists have theorized that this may be due to crowding indoors, decreased human host immunity, or environmental factors during the colder months. Drs. Peter Palese et al* used guinea pigs to prove that prevailing temperature and relative humidity are critical to the flu's successful spread.

They found that infected guinea pigs most easily shared influenza with their colleagues when housed in cooler labs (41 degrees F) at the relatively low humidity of 25-30%. The sickened pigs shed the virus for 40 hours longer at this level than when the lab temperature was raised to 68 degrees, and no well pigs sickened when the thermostat was set at 86 degrees. The investigators found no difference in the animals' immune response at the varying temperatures suggesting that the virus itself had better survival and transmissibility under cold and dry conditions.

So do we lower indoor temperatures and jack up the humidity to stay well through winter? Cooler temperatures may be healthier, but microbiologist Dr. Raymond Tellier worries that raising humidity may raise crops of other bad actors for human health, noting: "You don't want to reduce one infectious disease only to increase another."

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*Lowen AC, Mubareka S, Steel J, Palese P (2007) Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature. PLoS Pathog 3(10): e151 doi:10.1371/journal.ppat.0030151