Winters have shorter days and fewer hours of sunshine, which means less Vitamin D. This, in turn, reduces your immunity and makes you more susceptible to colds and other infections. In winters, people mostly stay indoors and are in closer contact with each other thereby increasing the risk of getting a cold. Moreover, indoor air is mostly dry in the winters due to use of heaters. This can reduce moisture in the respiratory tract and make it more susceptible to the cold viruses.