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.