My Husband Has Had Flu Like Symptoms With Upset Stomach

my husband has had flu like symptoms with upset stomach some vomiting and runs. He started out with he said yellow stools (brownish) and now have turned black like water in texture. He said he has some cramping feel in his stomach, no stabbing pains, temp was 98.9 yesterday and has been going on for (this is ) four days. what is your thought--is it flu or do I need to be concerned with something else?