Active immunization involves administration of vaccines containing antigenic molecules (or genes for these molecules) derived from infectious agents. As a result, vaccinated animals mount acquired immune responses and develop prolonged, strong immunity to those agents. When properly used, vaccines are highly effective in controlling infectious diseases. Several criteria determine whether a vaccine can or should be used. First, the actual cause of the disease must be determined. Although this appears self-evident, it has not always been followed in practice. For example, Mannheimia haemolytica can be isolated consistently from the lungs of cattle with respiratory disease; however, these bacteria are not the sole cause of this syndrome, and vaccines against the primary viral pathogens are required for full protection. In some important viral diseases (eg, equine infectious anemia, feline infectious peritonitis, and Aleutian disease in mink), antibodies may contribute to the disease process, and vaccination may therefore increase disease severity.