The BK virus, a human polyomavirus, is prevalent in much of the population. Many people excrete the virus in urine without having viremia. People at risk for BKVAN include those who are immunocompromised or immunosuppressed.
Garces, J.C. (2010). BK virus-associated nephropathy in kidney transplant recipients. The Ochsner Journal, 10:4, 145-249. Retrieved from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3096223/