Polyomavirus infection occurs in the transplanted allograft kidney and in the native kidney of immunosuppressed patients, and is usually related to BK virus (80%), JC virus (10%-20%), and less frequently simian virus 40 (SV40). Polyomaviruses are DNA and nonenveloped, with tropism for the urothelium. Transplanted kidneys have about 5% infection prevalence. Polyomavirus nephropathy usually presents as an asymptomatic acute or slowly rising creatinine. Polyomavirus infection can also present as nonhemorrhagic or hemorrhagic cystitis, and rarely as ureteral obstruction.