Death of the conceptus probably results from the collective damage by the virus to a variety of tissues and organs, including the placenta. However, in the absence of an immune response, changes in almost any vital organ are probably sufficient to eventually cause death. One of the most striking features of viral distribution is the extensive involvement of endothelium. This seems to preclude further development of the vascular network of the conceptus. Preparation for cellular mitosis (i.e., the S phase) results in concomitant viral replication and cell death. Damage to the fetal circulatory system is indicated by edema, hemorrhage, and the accumulation of large amounts of serosanguineous fluids in body cavities. Necrosis of the endothelium is microscopically evident.