Methods for continuously inactivating a virus during manufacture of a protein are provided. Steps include (1) combining, at a single predetermined volumetric ratio, a composition including the protein, and a composition including a virus-inactivation reagent, to obtain a treatment composition having a predetermined property for inactivation of a virus; (2) transferring the treatment composition to a treatment vessel; (3) incubating the treatment composition in the treatment vessel at predetermined conditions; and (4) subjecting the treatment composition to a post-treatment processing operation. The single predetermined volumetric ratio has been selected to ensure that the predetermined property of the treatment composition is maintained across a range of concentrations predicted for the protein in the composition including the protein. Steps (1) to (3) are carried out (a) continuously and (b) without feedback control being exerted by either adjustment of the volumetric ratio or diversion of the treatment composition.