Continuity mistake: While Dr. Harvey is looking at his multiple reflections in the mirror over the sink, he sees the Crypt Keeper reflection. It then cuts to a close-up facing the real Dr. Harvey as he freaks out, and it's a flipped shot—note his shirt and sweater are backwards (this is not a mirror's reflection)—right before he gets his foot jammed in the bucket. (00:31:05)
Chosen answer: It was most likely Pneumonia, which was very deadly before the discovery of antibiotics.
Donald Jenkins