Vitamin D synthesis drops significantly in winter because sun exposure decreases. Northern India in particular sees meaningful vitamin D insufficiency between November and February. Brief midday sun exposure of 15 to 20 minutes is beneficial, or discuss supplementation with your doctor.
Respiratory viruses peak in cold months. Dry indoor air from heaters dries out mucous membranes that act as a first barrier against infection. Warm clothing worn in layers is more effective than a single heavy layer for maintaining body temperature.