As long as flu has been recognized, people have asked, Why winter? The very name, “influenza,” is an Italian word that some historians proposed, originated in the mid-18th century as influenza di freddo, or “influence of the cold.”Over the years, many hypotheses have been invoked to explain this seasonality, among them, winter crowding indoors, reduced immune efficiency, lower vitamin D levels due to lack of sunlight, lower melatonin levels due to shorter days, and even upper atmosphere air currents. But none have survived scientific scrutiny. Now, Kolata writes, a new study with guinea pigs appears to have pinned down the real reason for the flu bug's fondness for winter. It turns out that the virus is more stable in cold air, and that it remains airborne longer in dry air.
Flu season in northern latitudes is from November to March, the coldest months. In southern latitudes, it is from May until September. In the tropics, there is not much flu at all and no real flu season.
By varying air temperature and humidity in the guinea pigs’ quarters, they discovered that transmission was excellent at 41 degrees. It declined as the temperature rose until, by 86 degrees, the virus was not transmitted at all.So, if you're worried about catching the flu, should you just stay indoors during flu season and crank up the humidifier? No. Peter Palese, the lead author of the study and chairman of the microbiology department at Mount Sinai School of Medicine in New York, recommends flu shots.
The virus was transmitted best at a low humidity, 20 percent, and not transmitted at all when the humidity reached 80 percent.
The animals also released viruses nearly two days longer at 41 degrees than at a typical room temperature of 68 degrees.
Flu viruses spread through the air, unlike cold viruses, Dr. Palese said, which primarily spread by direct contact when people touch surfaces that had been touched by someone with a cold or shake hands with someone who is infected, for example.
Flu viruses are more stable in cold air, and low humidity also helps the virus particles remain in the air. That is because the viruses float in the air in little respiratory droplets, Dr. Palese said. When the air is humid, those droplets pick up water, grow larger and fall to the ground.