Which diseases or illnesses cause diarrhea?

Lots: Crohns disease, ulcerative colitis, microscopic colitis, celiac disease, pancreatic insufficiency, bacterial infection, parasitic infections, viral infections, medications and irritable bowel syndrome as a brief list. There are entire textbooks on the subject.

Diarrhea: There are so many potential causes of diarrhea. If you have had persistent diarrhea i recommend a full physical with lab testing from your primary physician.