Patients receive a physical examination during visit 1. A stool sample, blood sample, and 6 biopsies are collected at visit 2, and patients undergo a flexible fiberoptic sigmoidoscopy. Patients receive idelalisib orally (PO) twice daily (BID) after visit 2. Treatment continues in the absence of disease progression or unacceptable toxicity. A third research visit occurs upon development of idelalisib-associated diarrhea/colitis symptoms. Patients with diarrhea/colitis symptoms undergo a full colonoscopy and collection of stool and blood samples. Control patients with no diarrhea/colitis symptoms undergo all needed tests and assessments including a flexible fiberoptic sigmoidoscopy and collection of stool and blood samples. All patients undergo optional biospecimen collection at the time of disease progression.