There is a correlation and a cause-effect between being wet, hot, cold, and sick. The key terms here are temperature differential, temperature drop, and temperature shock. The way it was explained to me by one of the very few doctors I consider competent: Our body is constantly running our immune system, like a factory, using calories as power source, and stomach as a furnace. Immune system constantly wards off various bacteria and virus strains. Most of the time, day to day, we do not notice this, because our body runs the defenses just fine. Normally our body can deal with things like wind, extreme heat, and rain, for some periods of time. There isn't a strict science or formula to establish exact connection, because every person has different immunity capacity and different stomach. But the way it works is: being exposed to wind while wet, increases our temperature drop, and makes our body work harder, straining the immune system. Thus, rain can cause sickness. Being exposed to sudden temperature differentials and fluctuations can do the same thing. Every time I fly from Alaska to Seattle, in the summer, I get sick, because my body is used to the adjustment mode it has in Alaska weather. When in a hot climate, my body now has to work extra to cool itself, and the immune system suffers the same. Voila, I catch a cold in 95 degrees F. I catch a hot really, not a cold. LOL. The third is temperature shock: diving into an ice filled bath tub, then blowing yourself over with a hot fan, then eating an ice cream, and drinking hot tea, and then laying down with air conditioner on - produces a lot of tension in the body, as it has to constantly adjust. BAM, you get sick next morning. TL;DR: Wet, Cold, and Hot all do the same thing, they force your body to redirect its calorie energy to maintaining proper temperature, instead of running the immune system, which ends up getting you sick.