idiopathy

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A disease of unknown origin or cause.
• noun A primary disease arising spontaneously with no apparent external cause.

from The Century Dictionary.

• noun In pathology, an idiopathic character of disease; a morbid state or condition not preceded and occasioned by any other disease.
• noun An individual or personal state of feeling; a mental condition peculiar to one's self.

from the GNU version of the Collaborative International Dictionary of English.

• noun A peculiar, or individual, characteristic or affection.
• noun (Med.) A morbid state or condition not preceded or occasioned by any other disease; a primary disease.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Something idiopathic; a disease having no known cause.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any disease arising from internal dysfunctions of unknown cause