I've read that the cold dry weather can dry out the cilia (tiny little hairs) in the respiratory tract that kind of wave constantly for the purpose of escorting germs and allergens out. And if they break or otherwise malfunction more allergens remain in the respiratory tract during winter and cause problems.

Also, there are thousands of rhino viruses (common cold germs) that are more easily caught in winter when they can circulate in closed houses and offices. They can manifest in serious flu type symptoms or in minor allergy type symptoms and it's difficult to tell which is which.

Your husband, being close to the faces of his patients is probably much at risk for various viruses.

smile