Influenza di freddo, or “influence of the cold.”

Study Shows Why the Flu Likes Winter

Health / Research
Study Shows Why the Flu Likes Winter
By GINA KOLATA
Published: December 5, 2007
Researchers believe the flu virus is more stable and stays in the air longer when air is cold and dry.

Excerpt:

Flu viruses spread through the air, unlike cold viruses, Dr. Palese said, which primarily spread by direct contact when people touch surfaces that had been touched by someone with a cold or shake hands with someone who is infected, for example.

Flu viruses are more stable in cold air, and low humidity also helps the virus particles remain in the air. That is because the viruses float in the air in little respiratory droplets, Dr. Palese said. When the air is humid, those droplets pick up water, grow larger and fall to the ground.

Comment: My Father (b 1918), and Kathee's parent's (Father (b. 1913), Mother (b. 1917)) all survived the 1918 influenza outbreak. This is a good read!