I'm in the midst of being taught a valuable biking lesson - my skin is not invincible.
Months of riding with a balaclava, tossing it down after the ride, never washing it...then a week of very rainy weather, soaking the balaclava, wearing a soaked balaclava while biking and still never washing it...
So, now I have a fungal skin infection. Initially, I thought that it was contact dermatitis, but after seeing it spread a bit from the back of my neck despite no further stimuli, it definitely seems to be fungal. I do not recommend this approach.