Clark, who remained in the clubhouse dressed in street clothes while his teammates took batting practice, said he definitely will play in Game 3 Tuesday at Candlestick Park.
Asked if he might miss the game, Clark replied there were only two things that would keep him out: "I've either gotta be in the hospital or be dead."
Clark, who went 0-for-4 in Sunday's 5-1 loss that gave Oakland a 2-0 lead in the Series, said he has been feeling bad for a couple of days.
"It's a little of everything," he said of the illness. "It started out as tonsillitis, and it's gotten worse."