What Causes Headache,body Ache,cramping And Diarrhea?

My son went to El Salvador. He got back and is having headaches, body aches, cramping & diarrhea. 30 kids were on the trip and and they are all sick. We parents are not finding the solution at any of the drs. so far. We had the urine checked today, it was fine. We did a stool sample too. Others are saying it might be billierium. Almost all the others were on cipro and not getting better, they are worse. They gave me sulfameth, metronidazole and diphen. Are we on the right track to get this resolved?