Cleveland ? A viral infection is the likely cause of the stomach distress that sent former President Jimmy Carter to a Cleveland hospital after a flight from Atlanta, medical staff said Wednesday.

Carter, who was spending a second night in the hospital, was recovering and the viral infection was clearing up, said a statement from MetroHealth Medical Center.

“President Carter thanks all those who have expressed concern and sent greetings to him,” the statement says.

The 85-year-old ex-president became ill Tuesday during the Delta Air Lines flight, causing rescue crews to rush him to the hospital after the plane landed. He was whisked from an airplane to the facility’s emergency room, and was placed under round-the-clock monitoring by a team of doctors.