According to a research by The National Institute of Health, the flu virus’ coating becomes tougher at low temperatures close to zero. This makes the viruses stronger and more resilient. See also how to get over a cold fast.
One theory suggests that cold weather inhibits the ability of mucus and nasal hairs to prevent diseases from getting inside your system. When this happens you become more likely to get a cold.
In extreme cold weathers, the body can lose much heat, and as a result, the functions of the major organs might be negatively affected. See how to survive extreme cold weather.
According to a research at Yale University, lower temperatures weaken the nose’s first line of immune defenses. It was found that at lower temperatures, the body's ability to fight infections gets lower. See also why your body is always cold.
According to studies, infected cells inside the body self-destruct to prevent further infection. In cold weather, this is less likely to happen and so the virus is more likely to spread in the body.
Vitamin D helps our bodies fight colds. During winter we don't get enough sunlight which is an essential factor for the manufacturing of Vitamin D inside our bodies.
Researchers found that cold virus can reproduce more efficiently in the cooler temperature of our nasal passages. This is another reason why we are more likely to get colds in winter. See why eggs can make you feel sick.
Dry winter air allows the flu virus to survive and transmit itself. It's not the cold air that causes sickness, but it's the dryness of the cold weather that does so. See also why am I always sick.
According to one theory, cold air forces people to stay inside buildings that have dry air. The dry air thus increases the chances of viruses reaching your dry nasal passages. The dry air itself doesn't cause a cold but it helps the viruses spread easily.