I’ve been regularly swimming in cold water (last week it was 25 minutes in 9 degree water). There has been some research that suggested that extreme cold adaptation might improve your immune system response. I am experimenting with whether this is true.
As I understand it, the thing about cold viruses is that if the germs get into your nose or wherever, you have a 95% chance of catching them, pretty much independent of how strong your immune system is. Hence why us with small kids tend to get quite so many colds no matter what.
So the only measure is how long it takes to get rid of them.
In the last month, I’ve had three colds, which seem to have lasted less than a week each, with gaps in which I have swum, done a running race, biked to work etc.
So I haven’t had the long stretches of no exercise that I had last year, but I have had a bunch of colds. So I’m currently a bit inconclusive on my experiment. It certainly feels jolly good to jump in the river in winter though, so I guess I’ll keep doing it whatever.