In Russia it is very visible that colds and flus are most widespread in autumn, secondly in spring and when it gets warmer than usual in winter. Let's say, about plus ten to minus two. Hardly anyone gets a cold at minus twenty. People explain it by saying that the viruses can't survive in very cold air, but I find this doubtful because almost all human interaction takes place indoors at those times.
What explanations I could imagine to be right:
1) the viruses can't survive in very dry air. It is well konwn that very cold air gets very dry as soon as you heat it (breathe it in or let it into the house).
2) the cold itself doesn't strain the immune system but rather strengthens it - it's the change that is hard to bear, especially the change from summer (one temperature everywhere) to autumn (big difference between indoors and outdoors). That would also explain why you often get a cold in summer when you spend time in an air-conditioned house or car, not being used to it.
3) Our grandmothers were right after all - the worst thing is getting wet feet...
