Teresa Heinz Kerry Teresa Heinz Kerry

Teresa Heinz Kerry, the wife of Secretary of State John Kerry and the heir to the ketchup company fortune, suffered a grand mal seizure that sent her to the hospital on Sunday, a medical source who treated her tells NBC News.

Heinz Kerry, 74, was upgraded to fair condition on Monday after she was described as being in "critical but stable" condition Sunday when she was rushed to a hospital in Nantucket and then later transferred to Massachusetts General Hospital in Boston. Heinz Kerry had to be intubated and continues to undergo evaluation.

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Also known as a generalized tonic-clonic seizure, a grand mal seizure is described as "an electrical storm in the brain." It is the most serious type and normally involves the entire body, causing the victim to lose consciousness. It is also unclear what could have caused Heinz Kerry's seizure, which can be prompted by a stroke, an underlying medical condition or even serious sleep deprivation.

Heinz Kerry underwent treatment at Massachusetts General for breast cancer in 2009. She has three adult sons from her first marriage to the late Sen. John Heinz, who died in a plane crash in 1991.

Kerry, her husband since 1995, has been by her side throughout her hospitalization. "The family is grateful for the outpouring of support it has received and aware of the interest in her condition, but they ask for privacy at this time," Kerry's spokesperson said in a statement on Sunday.