Pigs nursing immune dams absorb a high titer of antibody for PPV from colostrum. These titers decrease progressively with time by dilution as pigs grow as well as by biological degradation. They usually reach subdetectable levels in 3–6 months if sera are examined by the HI test. Sometimes passively acquired antibody persists for a longer interval. Moreover, levels of antibody too low to be detected by the HI test may be detected by the SN test. The primary significance of passively acquired antibody is that it interferes with the development of active immunity. High levels of such antibody can prevent infection, and lower levels can minimize dissemination from infected pigs. Consequently, some groups of gilts are not fully susceptible to infection and dissemination of virus until either shortly before conception or during early gestation.