All kidney transplant recipients are routinely assessed for BK virus nephropathy and viremia post transplant as part of their standard care. Patients with BK viremia or BK virus nephropathy will have adjustments in their immunosuppressive medication based on current guidelines and recommendations which include reduction in immunosuppression, treatment with medications with antiviral activity (Cidofovir, Ciprofloxacin, Leflunomide, IVIG) or switching from a Tacrolimus (Prograf)-based regimen to a Cyclosporine-based regimen. Retrospectively, the investigator will collect data on patients who have been diagnosed with BK viremia or BK virus nephropathy and have had such a management in past 66 months (from 1/1/2010 till 06/30/2015). Prospectively, the investigator will enroll and collect data on patients who will be treated for BK viremia or BK virus nephropathy.