Re­search­ers say they’ve solved a great mys­tery of the flu: why it spreads mainly in win­ter. The an­swer, they re­port, is that the vi­rus is more stable and re­mains air­borne long­er in the cold, dry air that pre­vails in the sea­son.

A typ­i­cal flu vi­rus is shown "s­liced" in half in this di­a­gram to show the in­sides. It con­tains genes for mak­ing cop­ies of it­self on the in­side, and "spikes" that help it at­tach to a host cell on the out­side. (Cour­te­sy Nat'l In­sti­tute of Al­ler­gy and In­fec­tious Dis­eases, U.S.)

The New York Times re­ported the find­ings, al­so pub­lished in the Oct. 19 is­sue of the re­search jour­nal PLoS Path­o­gens, on Wednes­day.

“In­fluenza vi­rus is more likely to be trans­mit­ted dur­ing win­ter on the way to the sub­way than in a warm room,” Pe­ter Palese, cha­irman of mi­cro­bi­ol­o­gy at Mount Si­nai School of Med­i­cine in New York and the stu­dy’s lead au­thor, told the Times.

Palese con­ducted the study af­ter read­ing a pa­per pub­lished in the af­termath of the dev­as­tat­ing 1918 flu pan­dem­ic. That re­port sug­gested the vi­rus could rap­idly spread among guin­ea pigs.

Palese and col­leagues ex­pe­ri­mented with flu vi­rus among guin­ea pigs, var­y­ing air tem­per­a­ture and hu­mid­ity in the an­i­mals’ quar­ters. They found that trans­mis­sion was best at 41 de­grees Fahr­en­heit (5 de­grees Cel­sius), and wors­ened at high­er tem­per­a­tures, end­ing com­pletely at 86 de­grees Fahr­en­heit (30 de­grees Cel­sius). 

Trans­mis­sion was al­so best at a low hu­mid­ity of 20 per­cent, Pa­lese said. That’s probably be­cause the vi­ruses float in the air in lit­tle res­pi­ra­to­ry droplets, he added. In hu­mid air, the droplets grow larg­er and fall.

Flu vi­ruses spread through the air, Pa­lese said, where­as cold vi­ruses spread mainly by di­rect con­tact, such as hand­shakes or con­tact with an ob­ject just touched by an in­fected per­son. Flu sea­son in north­ern lat­i­tudes runs from No­vem­ber to March, the cold­est months. In south­ern lat­i­tudes, it’s from May un­til Sep­tem­ber. In the trop­ics, there is lit­tle flu at all.

Flu re­search­ers told the Times they were de­light­ed to fi­nally get some sol­id da­ta on flu sea­sonal­ity. “It was great work, and work that needed to be done,” said Ter­rence Tumpe, a sen­ior mi­cro­bi­ol­o­gist at the U.S. Cen­ters for Dis­ease Con­trol and Pre­ven­tion.