 Infection after approximately day 120 can result in the birth of a normal fetus which is BVD antigen-negative and BVD antibody-positive.This occurs because the fetal immune system has developed, by this stage of gestation, and has the ability to recognise and fight off the invading virus, producing anti-BVD antibodies.BVD virus can be maintained as a chronic infection within some immunoprivileged sites following transient infection.