BK virus nephropathy is a kidney condition caused by the BK virus, a common virus that usually remains dormant in healthy individuals. In people with weakened immune systems, such as kidney transplant recipients, the virus can reactivate and lead to kidney damage. This condition is characterized by inflammation and injury to the kidney tissue. Symptoms of BK virus nephropathy may include decreased kidney function, increased blood pressure, and changes in urine output. Diagnosis typically involves blood tests, urine tests, and kidney biopsies. Early detection and management are crucial to prevent severe kidney damage and ensure the health of the transplanted kidney.