“We know one of the largest factors is kids in school; most of the major epidemics are traced to children,” said Dr. Jonathan McCullers, a flu researcher at St. Jude Children’s Research Hospital in Memphis. “But that still does not explain wintertime. We don’t see flu in September and October.” As for the crowding argument, McCullers said, “That never made sense.” People work all year round and crowd into buses, subways and airplanes, no matter what the season. Reading a paper published in 1919 in the Journal of the American Medical Association on the flu epidemic at Camp Cody in New Mexico, he came upon a key passage: “It is interesting to note that very soon after the epidemic of influenza reached this camp, our laboratory guinea pigs began to die.” At first, the study’s authors wrote, they thought the animals had died from food poisoning. But, they continued, “a necropsy on a dead pig revealed unmistakable signs of pneumonia.” Palese bought some guinea pigs and exposed them to the flu virus. Just as the paper suggested, they got the flu and spread it among themselves. So Palese and his colleagues began their experiments. By varying air temperature and humidity in the guinea pigs’ quarters, they discovered that transmission was excellent at 5 degrees Centigrade, or 41 degrees Fahrenheit. It declined as the temperature rose until, by 30 degrees Centigrade, or 86 degrees Fahrenheit, the virus was not transmitted at all. 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 Fahrenheit than at a typical room temperature of 20 degrees Centigrade, or 68 degrees Fahrenheit.160Want local news?Sign up for the Localist and stay informed Something went wrong. Please try again.subscribeCongratulations! You’re all set! AD Quality Auto 360p 720p 1080p Top articles1/5READ MOREPettersson scores another winner, Canucks beat KingsAs 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-eighteenth century as “influenza di freddo,” or “influence of the cold.” 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. There was no shortage of hypotheses. Some said flu came in winter because people are indoors; and children are in school, crowded together, getting the flu and passing it on to their families. Others proposed a diminished immune response because people make less vitamin D or melatonin when days are shorter. Others pointed to the direction of air currents in the upper atmosphere. But many scientists were not convinced. Researchers in New York believe they have solved one of the great mysteries of the flu: Why does the infection spread primarily in the winter months? The answer, they say, has to do with the virus itself. It is more stable and stays in the air longer when air is cold and dry, the exact conditions for much of the flu season. “Influenza virus is more likely to be transmitted during winter on the way to the subway than in a warm room,” said Dr. Peter Palese, a flu researcher who is professor and chairman of the microbiology department at Mount Sinai Medical College in New York and the lead author of the flu study. Palese published details of his findings in the Oct. 19 issue of PLoS Pathogens. The crucial hint that allowed him to do his study came from a paper published in the aftermath of the 1918 flu pandemic, when doctors were puzzling over why and how the virus had spread so quickly and been so deadly.