What Causes Severe Diarrhea?

My husband was feeling bad last week and ended up shaking real bad in bed this past Saturday night. He had diarrhea extremely bad, too. He went to his Dr. today because he seemed to be doing better yesterday until he ate food and now he is even worst. They couldn't take blood because he was too dehydrated. I'm not sure if we should go have him checked out further or if it could be a virus.