Anecdotal evidence: in the past, when I got a cold it settled in my chest, and I was left with a cough that went on for at least 10 days, mostly longer.
Two years ago I was one day into a bad cold when, for reasons too boring to go into, I got called upon to put in a four hour stretch of genuine hard labor in an overheated setting. Lifting and hauling and shoving furniture around and such – quite a change from my normal desk job. I felt punk when I started, and punk & horribly hot all day, and unbelievably sore when I went to bed that night…but when I woke up, the cold was gone. Not even a sniffle.
I thought this was amazing, though possibly coicidental. So the next time I came down with a cold I deliberately set out to duplicate the ‘physical labor’ cure. I didn’t have an hot location to work in, but it was a warm Indian Summer day and I spent it raking and bagging leaves and the trillions of useless-because-they-are-diseased apples that the aged remnants of an apple orchard drop in our back yard each year. Result: the cold wasn’t completely gone the next morning, but it was vastly improved. Like jumping from day 1 to day 5 of a cold, skipping the worst of it.
The next cold I was feeling lazy, so I just went to my health club and hung out in the steam room for a few hours. Result: no effect on my cold at all.
So…I guess it isn’t just being hot, it has to be internally generated heat or something.