BK Virus/physiology; Polyomavirus Infections/virology; Tumor Virus Infections/virology; Antiviral Agents/therapeutic use; BK Virus/genetics; BK Virus/pathogenicity; Carrier State/epidemiology; Carrier State/virology; Cell Transformation, Viral; Female; Humans; Immunocompromised Host; Immunoglobulins, Intravenous/therapeutic use; Immunosuppressive Agents/adverse effects; Immunosuppressive Agents/therapeutic use; Male; Meningoencephalitis/diagnosis; Meningoencephalitis/virology; Neoplasms/virology; Nephrectomy; Polyomavirus Infections/diagnosis; Polyomavirus Infections/drug therapy; Polyomavirus Infections/epidemiology; Polyomavirus Infections/immunology; Postoperative Complications/diagnosis; Postoperative Complications/drug therapy; Postoperative Complications/virology; Transplantation; Tumor Virus Infections/diagnosis; Tumor Virus Infections/drug therapy; Tumor Virus Infections/epidemiology; Tumor Virus Infections/immunology; Urinary Tract Infections/diagnosis; Urinary Tract Infections/virology; Virus Activation; Virus Latency