When I was a kid, I was chilling in the water of the Mediterranean Sea in Turkey. Suddenly I felt an awful burning sensation on my stomach and my legs.
I looked like I had been brutally sandpapered and I got a 40°C (104°F) fever. Turns out I made contact with a jellyfish, and later found out that it hadn’t happened on that beach for 10 years or so. I was just extremely unlucky.