Brief Summary: This trial studies the role of the gut microbiome and effectiveness of a fecal transplant on medication-induced gastrointestinal (GI) complications in patients with melanoma or genitourinary cancer. The gut microbiome (the bacteria and microorganisms that live in the digestive system) may affect whether or not someone develops colitis (inflammation of the intestines) during cancer treatment with immune-checkpoint inhibitor drugs. Studying samples of stool, blood, and tissue from patients with melanoma or genitourinary cancer may help doctors learn more about the effects of treatment on cells, and help doctors understand how well patients respond to treatment. Treatment with fecal transplantation may help to improve diarrhea and colitis symptoms.

Detailed Description: PRIMARY OBJECTIVES: I. To compare the difference in stool microbiome pattern between patients who develop immune-checkpoint inhibitor (ICPI)-related colitis and patients who don't develop ICPI-related colitis. II. To compare the difference in stool microbiome pattern in patients who developed ICPI-related colitis before and after colitis medical treatment. III. To assess the safety and tolerability and efficacy of fecal microbiota transplantation (FMT). SECONDARY OBJECTIVES: I. To identify and characterize immune profile and genetic factors associated with onset of ICPI-related colitis in blood and colon tissue. II. To identify and characterize immune profile and genetic factors in blood and colon tissue that are associated with quick response of ICPI-related colitis to medical treatment. III. To characterize the endoscopic and histologic features of ICPI-related colitis before and after medical treatment. IV. To document the changes of ICPI-related symptoms and the impact on functioning and quality of life (QoL) from fecal microbiota transplantation by patient-reported outcomes (PRO). V. To assess stool microbiome and cytokine features that are associated with good response to fecal microbiota transplantation. VI. To assess the factors in genetic/immune profile obtained from blood and colon tissue that are associated with good response to fecal microbiota transplantation. VII. To characterize the endoscopic and histologic features of ICPI-related colitis before and after fecal microbiota transplantation. EXPLORATORY OBJECTIVES: I. To identify and characterize immune profile and genetic factors associated with onset of ICPI-related colitis in inflamed colonic mucosa and its matched normal mucosa. II. To characterize the immune profile and genetic factors from the colon tissue in these colitis patients among different histological subtypes. III. To assess the pattern of stool microbiome that is associated with good tumor response to ICPI treatment. IV. To assess the association between stool inflammatory markers (i.e. lactoferrin and calprotectin) and the severity of endoscopic/histologic inflammation. V. To assess the sensitivity and specificity of stool inflammatory markers (i.e. lactoferrin and calprotectin) as an indicators of ICPI-relate colitis response to treatment. VI. To assess the microbiome pattern that triggers the infections on immunosuppressant treatment for ICPI colitis. OUTLINE: PROJECT 1: Patients receive standard of care and undergo collection of stool and blood samples. PROJECT 2: Patients receive prednisone, infliximab, or vedolizumab per standard of care and undergo standard of care endoscopy 2 months after treatment. Patients also undergo collection of stool, blood, and tissue samples. PROJECT 3: Patients undergo fecal microbiota transplant (FMT). After completion of study, patients are followed up periodically.