After kidney transplantation, patients are most worried about the hypofunction of the transplanted kidney. In addition to the common factors such as rejection and drug toxicity, viral infection is also an important cause of transplanted kidney decompensation. The BK virus is a virus that mainly attacks the transplanted kidney, usually “lurking” in the body, and once the “time is ripe”, it will attack the kidney. When the time is ripe, it will attack the kidney “without mercy”. Today, we will unveil the “true nature” of the BK virus. ”BK virus is a type of polyomavirus. After kidney transplantation, about 5% of patients will develop BK virus infection, mainly in the transplanted kidney and ureter, and the time of infection is mainly in the first year after kidney transplantation. ”BK virus infection mainly causes viral nephropathy in the transplanted kidney. Once the infection occurs, the transplanted kidney will fail to function in about 30-65% of patients. BK virus infection is usually detected in the patient’s urine (by PCR) and the presence of decoy cells in the urine can also be observed microscopically. However, the presence of “BK virus” replication and decoy cells does not necessarily mean that “BK virus” nephropathy has occurred. The presence of BK virus replication and decoy cells can sometimes be detected in the urine of normal subjects. In view of this, we have to further test the replication of BK virus in the plasma. If the replication of BK virus is present in both plasma and urine, the diagnostic significance is very strong. The best way to diagnose BK virus nephropathy, often called the “gold standard,” is a puncture biopsy of the transplanted kidney. The diagnosis can be confirmed by microscopic visualization of typical viral inclusion body formation in the kidney, as well as pathological changes of nephritis and interstitial tissue fibrosis in the transplanted kidney, and positive SV40 staining by immunohistochemical staining. Responding to BK virus infection Once a “BK virus” infection has occurred, the patient’s immune system has significantly decreased its resistance to the virus. The first priority at this point is to improve the patient’s immunity by reducing the use of immunosuppressive drugs, especially FK506 and primaquine, which are the most important ways to reduce the replication of the BK virus. Leflunomide, the antiviral drug cidofovir, and intravenous immunoglobulin can also be used. Recent studies have shown that the lipid-lowering drug pravastatin has been shown to inhibit BK virus damage to renal tubular epithelial cells in an in vitro cytology study, and is a viable treatment option. Since BK virus infection is usually silent and does not have typical clinical symptoms, tests for BK virus replication, such as PCR tests for BK virus in urine and blood, should not be neglected. Therefore, tests for BK virus replication, such as PCR tests for BK virus in urine and blood, should not be neglected, and should be repeated every three months during the first year after kidney transplantation and every six months thereafter. Once the BK virus is found to be “resurrected”, the type and dosage of immunosuppressants should be adjusted under the guidance of the doctor to effectively avoid the BK virus infection and protect the hard-won kidney. This will help to avoid BK virus infection and protect your kidneys.