Ross is in the hospital for a serious medical condition ... TMZ has learned. We're told Rick took ill in the Miami area at his home. Someone from the home called 911 at 3:30 AM Thursday and said the person in distress was breathing heavy and unresponsive. The caller said the man in distress had a history of seizures -- something that has plagued Rick -- and they were trying to wake him up but he was "slobbing out the mouth." According to law enforcement the person in distress came to and became combative. We're told Rick went to a hospital and is receiving a respiratory treatment, possibly for pneumonia. One source says Rick was asked to make an appearance Thursday night, but a rep told him the mogul was in the hospital. A family member strongly denied Rick was hospitalized. This isn't the first time Da Boss has had a major medical issue -- he suffered 2 seizures on 2 separate flights in October 2011 and was hospitalized. He blamed them on a lack of sleep at the time, but changed his lifestyle and diet afterwards ... and eventually dropped 100 lbs.