We were just talking about this at work Some say its because of the lack of vit. D. , Sun helps in making it , and winter being gloomy, its just not made enough. with out Vit. D , our immune system can be compromised easier. Along with the fact we dont spend alot of time outside. We cram ourselves into a warm house, were the temps are just about perfect for some virus to thrive. Some call it the "Daycare syndrome" Bring One sick child to a daycare...the domino effect is soon realized. Low temps and low humidity let the virus live longer and ease the possibility of transmission. High heat and humidity , you would think would be a breeding ground., but they theorize that the water droplets actually capture viruses and fall to the ground, reducing the chances of casual transmission.