WASHINGTON -- Although U.S. doctors can only speculate on Fidel Castro's health, his self-described symptoms could signal severe ulcers, a colon condition called diverticulosis or, an outside possibility, even cancer. Whatever the cause, major abdominal surgery in a soon-to-be 80-year-old is risky, made more so if Castro is indeed weakened from prolonged bleeding. Pneumonia, blood clots and other life-threatening complications can crop up in the critical first days of recuperation.