Idiopathic

id-i-o-path-ic* (d--pthk, d--)

adjective
1. Of unknown cause, pertaining to medical conditions.
Dr House fights idiopathic diseases by identifying cause.

Synonyms: idiopathetic,** random, unknown, unlucky.

* I fully admit that I like this word because it sounds like it should describe pathological idiocy.
** I promise this is also a real word! No, it doesn't mean idiotic beyond the point of sympathy.