Anti-BKV antibodies are present in 50-80% of the adult population. Primary infection occurs mostly during childhood. In most cases, it is asymptomatic or brings on an acute respiratory disease and then continues to the latent phase, which is characterized by the long-term presence of anamnestic IgG antibodies in serum. In latently infected people, the virus can repeatedly reactivate, or the patient can be re-infected by other BKV serotypes. Reactivation/reinfection can be accompanied by temporary viremia or viruria. In immunodeficient people, it can cause various diseases of urinary tract (haemorrhagic cystitis, urethral stenosis), kidney (BK-viral nephropathy), central nervous system (encephalitis, polyradiculoneuritis), lung (intersticial pneumonitis) or vasculitis. Absence of anti-BKV antibodies may indicate a patient's susceptibility to primary infection, which can increase complication risks. Primary infection can be diagnosed using anti-BKV IgG seroconversion. A significant increase in antibody levels in paired serum/plasma samples can indicate reinfection or virus reactivation.