A washer disinfector (1 ) for disinfecting or sterilizing a device (D), comprises a washing chamber (2) which is adapted for housing said device (D), and an arrangement (34, 36, 38, 40, 42, 44) for supplying a haloperoxidase, hydrogen peroxide, chloride and/or bromide ions, and ammonium ions to said washing chamber (2). A method of disinfecting or sterilizing a device (D) in a washer disinfector (1 ) comprises supplying a haloperoxidase, hydrogen peroxide, chloride and/or bromide ions, and ammonium ions to the washer-disinfector (1 ).