probably from their thighs rubbing together, would that cause a boil? I'd assume the skin rubbing on skin would cause a rash or blister, like when you're doing a long backpacking trip and your backpack isn't adjusted right and it causes rashes in areas around your back/armpits where it's rubbing for hours on end. Or blisters from hiking 15+ miles a day for a week in new shoes. Can you tell I learned a lot of hard lessons on my first backpacking trip :P now I know to carry mole skin for blisters, and to make sure my pack is adjusted very well, and re-adjusted whenever the load shifts as I repack and go through supplies.