[Joe 6-pack]

During warm weather, a lot of people spend a lot more time outside in the yard, at the park, at the beach, etc. When you think about how much more time is spent by more people indoors during cold weather, it simply stands to reason that a higher percentage of people spend a higher percentage of their time in close proximity to one another in cold weather, making it much more likely that an airborne virus like the flu will have a greater opportunity of spreading and infecting more people.

[TigerLikesRooster]

I am sure that is a contributing factor, too. Still this article says that cold weather alone can raise the susceptibility significantly even if people does not have more cases of close contact with others.