I noticed even as a child that wet winters brought colds and flu s, but when the summer came, the threat was much less. Some people thought that this was due to the temperature, but my guess was always based on the moisture content of the air being a more suitable environment for the germs to survive and thrive and generally lower temperatures. That view seems to be correct:
https://www.naturalnews.com/2018-05-16-science-explains-why-we-are-more-vulnerable-to-getting-sick-when-the-seasons-change.html
“People are more vulnerable to getting sick when seasons change, especially during spring and fall, and experts may now know why. According to research, it’s not the cold air that makes people sick; rather, it’s the virus. Rhinoviruses and coronaviruses, the two main causes of the common cold, can multiply quickly when the weather is cool – but not too cold. “A lot of the viruses that cause the common cold are shown to cause outbreaks more frequently in the early to late spring and early to late fall,” explained Dr. Kittu Jindal Garg, an internal medicine specialist, in an interview with The Weather Channel. In the same way, the influenza virus multiplies and spread more easily and rapidly when the air is cold and dry. As a result, people tend to catch the flu during winter.
