Five to twenty percent of Americans catch the flu around late fall or winter. Both the cold and the flu are caused by viruses, most commonly Rhinovirus, Coronavirus, and H. Influenza. A recent Harvard Medical School newsletter article linked these combinations to increased cases of illness during winter:
1. People spending more time indoors – leading to 
more exposure opportunities for viruses to spread.
2. Less humidity – enables viruses to spread more readily and dryer noses lead to less protection from pathogens.
3. Vessel constriction in response to cold climate – slows immune response. Yes, dressing warm outdoors can combat this.
4. Less vitamin D from the sun on shorter days and more time indoors contributes to a lack of absorption of vitamin D from the sun which has adverse effects on our immune system.
