Doctors say Chief Justice John Roberts had a benign idiopathic seizure. It's a seizure where there is no underlying physiological cause. Roberts had a similar episode 14 years ago. The National Institutes of Health says once a person has had two or more seizures, he is considered to have epilepsy.

The Epilepsy Foundation says more than 3 million Americans have some form of the disorder, and about 200,000 new cases are diagnosed each year. There is no known cause in about 70% of cases.