Ileitis is inflammation of the ileum — the final, longest section of the small intestine, where vitamin B12, bile salts, and remaining nutrients are absorbed. Crohn’s disease is its most important cause: the transmural inflammation classically strikes the terminal ileum, producing cramping pain, diarrhoea, weight loss, and — as absorption fails — B12 deficiency and malnutrition. Infections (Yersinia, tuberculosis), NSAIDs, and ischaemia cause it too. Diagnosis combines endoscopy or imaging with biopsy; treatment targets the cause — immunosuppression for Crohn’s, antibiotics for infection. Because the ileum is prime absorptive real estate, chronic ileitis is a nutritional disease as much as an intestinal one, managed jointly by gastroenterologists and dietitians.