Research done by the National Institute of Health suggests that the coating of a flu virus becomes tougher at temperatures close to freezing, making them more active, more resilient, and easier to transmit in the winter. Dry air also helps the flu virus to survive and transmit itself, and humidity is much lower in the winter. There is also another theory that being exposed to cold air can lower your body's ability to fight a virus. There are a few reasons for this. Because you are out in the sunshine less, vitamin D levels are usually lower in the winter. Vitamin D is thought to support your immune system, so low levels mean a less-supported defense. A more widely-accepted factor is constricted blood vessels. When you inhale cold, dry air, blood vessels in your upper respiratory tract constrict to conserve heat. That's fact. The theory developed through research is that narrowing may make it harder for white blood cells to reach your mucous membrane - the front lines of fighting off germs.

Aside from the virus itself, human behavior is another reason virus spreads more in the winter. When it's cold outside, people generally want to stay inside. With more people in confined spaces, you are sharing air and spreading germs more easily than in the summer when more people are outside.