The BK virus is a type of polyomavirus that commonly infects humans, often without causing any symptoms. It is typically spread through urine and can remain dormant in the body for long periods. In healthy individuals, the immune system usually keeps the virus in check. However, in people with weakened immune systems, such as those who have undergone kidney transplants, the BK virus can reactivate and lead to serious complications, including BK virus nephropathy, which can damage the transplanted kidney. Monitoring and managing the virus is crucial for the health of these patients.