id·i·op·a·thy 
(ĭd′ē-ŏpə-thē)
(ĭd′ē-ŏpə-thē)n.
1. A disease of unknown origin or cause.
2. A primary disease arising spontaneously with no apparent external cause.
[New Latin idiopathīa, primary disease, from Greek idiopatheia : idio-, idio- + -patheia, -pathy.]