A 38-year-old man with a history of fever associated with abdominal pain of 3-week duration presents now with a sudden onset of abdominal pain and copious vomiting. Plain abdominal x-rays reveal air under a diaphragm. A CT scan shows mesenteric lymphadenopathy and splenomegaly is found. Laparotomy is performed and 3 feet of ileum resected. The luminal aspect of the resected bowel shows marked ulceration of Peyer's patches. What is the most likely diagnosis?