BK virus is a human polyoma virus that infects the renal epithelium and remains latent until immunosuppression triggers reactivation. After reactivation, BK virus can be detected in the urine by methods currently available in the clinical laboratory. Correlations can be made between BK viruria and the occurrence of both renal and hepatic pathologies. BK virus is emerging as a significant pathogen in transplant patients. Additionally, the presence of BK virus DNA in primary brain and pancreatic tumors suggests that it may have oncogenic potential. Thus far, attempts to treat BK virus infection have been ineffective, though research has opened new avenues for treatment possibilities. Prevention of BK virus and other latent viral reactivation remains a challenge to viral research.