I know that article by the WHRU is tough going, I will be translating soon, based on Wizer's excellent suggestion. But to recap briefly, in order for the follicle/pore to turn into a Pilonidal, it first gets stretched and enlarged. The resulting "pit" becomes a magnet for hair and debris, which all of us know loves to collect in places like the buttock cleft, belly button, and between the toes, etc... As gunk gets sucked into this expanded pit, a foreign body reaction occurs, followed by infection. So, not every hair follicle can become a Pilonidal. Special considerations also need to be present, such as location, friction, pressure and possibly a limited ability of an individual to fight off minor infections. Some of us also manufacture more oils in our skin and are therefore prone to things like acne as well.