The BK virus is a virus that mainly infects the kidneys and urinary tract. Most people get infected with the BK virus sometime during their life.

Infection with the BK virus rarely causes symptoms, except in people who have a weakened immune system. Most of the problems from the BK virus are seen in people who have had an organ transplant. People who have transplanted organs must take medicines to prevent their bodies from rejecting the organs they receive. These medicines also lower their ability to fight infection. In these people, a BK virus infection can damage the kidneys.