What Causes Frequent Diarrhea?

I am a fresh MBBS graduate sir. My father had the complaint of frequent loose motions which even after several courses of antibiotics recurred frequently. I suggested for complete blood examination, stool examination, and occult blood test. The blood report showed wbc count as 11000, other parameters normal, the stool report showed only mucous and few pus cells, but occult blood test was positive! I went for colonoscopy, in which multiple ileal ulcers were seen,biopsy report says non specific ileitis, with zn stain negative, and tb pcr also negative . If you could please give your opinion sir, I would be grateful..