Here is just another alternative to give you some added information:

First, "spontaneous" is described as 自然に起こる、自然発生的、…(Shogakukan J-E Dictionary)

I agree with "unprovoked" and "spontaneous" being basically synonymously. Another word for it is “idiopathic.” Idiopathic is 特発性 so HappySmile has a great answer. Please see website below:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2518190/
Between these 2 extremes lie patients who have had a venous thromboembolism associated with a minor reversible risk factor (e.g., estrogen therapy or soft-tissue leg injury) and those who have had an “unprovoked” venous thromboembolism (also referred to as “idiopathic” or “spontaneous”).3

Difference between “provoked” and “unprovoked”….
http://neuro.wustl.edu/patientcare/clinicalservices/pediatri...
First of all, a distinction should be made between provoked and unprovoked seizures, since this has implications for treatment and prognosis, such as the risk of developing epilepsy. Provoked seizures are those events that result from some immediately recognizable stimulus or cause. Examples of such causes include hypoglycemia from excessive insulin in diabetic patients or fever in patients with febrile seizures. Provoked seizures may recur whenever the precipitating situation is present, but do not happen at other times in absence of the stimulus and thus do not constitute epilepsy. In contrast, unprovoked seizures do not require an immediate precipitating event. The occurrence of an unprovoked seizure suggests the possibility of an underlying neurological disorder that may predispose a child to recurrent seizures.
The evaluation of the first unprovoked seizure (FUS) must address several questions: Is this a seizure or some other paroxysmal event? Is this the first seizure? What are the characteristics of the seizure and does it represent a sign of a known syndrome or disease? How extensive should the evaluation be? Does the child need treatment with antiepileptic drugs (AEDs)? What is the risk of recurrence and long-term prognosis? This article is intended to overview the current approach to the evaluation and management of a first unprovoked seizure.

http://merckmanual.jp/mmpej/print/sec11/ch135/ch135a.html
自発的な深部静脈血栓の半数に及ぶ患者が，遺伝的素因を有する。






--------------------------------------------------
Note added at 13 hrs (2010-05-13 10:15:42 GMT)
--------------------------------------------------

Dear Ken-san,
Of course. I think HappySmile has the better answer so I cast my vote there, too.
But, I had collected all that information and I thought I'd better pass it on to you. :-) Thank you for your kind comments.