Its likely only correlated in some extent rather than a direct cause, but its a a belief held the world over from many unrelated cultures. In those cases, I'm willing to believe in old wisdom rather than assume all humans for thousands of years were just retarded.

Until science gets a direct answer of course instead of theories that are just as easily not the case a lot of the time.

Regardless, that's irrelevant to the point. I'm willing to guess their extreme ease of ear infection/inflammation from the wetness does a number on weakening/distracting their immune system to fight it and that in turn lets other otherwise weak or dormant infections come through.

But even if its not true, they certainly believe it heavily. And that's why its such a common trope for them, enough so that its reached the Vatican.