I’d never heard of the disease. Pilonidal Sinus is called the Jeep Disease or Jeep Seat (and probably others). I’ve taken the following paragraphs below from a Canadian link:
http://www.canada.com/topics/bodyandhealth/men/story.html?id=ac51b5e9-23a4-42c9-be8c-19a55aa0a8dc

“Pilonidal sinus most commonly affects that area known affectionately as the butt crack, though it can show up in the belly button, genital region, even between the fingers. Basically, it’s an infected area under the skin, frequently containing a tuft of hair. Think of it as a nasty, hairy, painful tunnel and you wouldn’t be far wrong. Often it starts with ingrown hairs, although new evidence shows that’s not always the case. There’s a genetic component; it tends to run in families, and it’s more prevalent among hairy guys, and those who sweat a lot.

According to a document co-authored by Prof. Keith Harding, the head of surgery at the University of Wales, the condition is 10 times more common in adult males than in adult females.

Harding notes that it was called “jeep disease” in the Second World War since it affected many soldiers who sat for long periods on hard seats.”

A couple more links:
From Wikipedia: http://en.wikipedia.org/wiki/Pilonidal_cyst
From the UK: http://www.wounds-uk.com/journal-articles/pilonidal-sinus-jeeps-disease-an-improved-model-of-care

John forwarded this meme.