Over the past 15 or 20 years our ideas concerning viruses and virus diseases have undergone some rather fundamental changes. In the past the term "virus disease" immediately implied an infection manifested by distinctive clinical signs and, in the n10re extreme interpretation, one characterized by an acute, and frequently fatal course. However, in the light of recent knowledge concerning latency and subclinical infection and the growing evide11ce that the host-parasite relationship is not static but an ever-changing complex, the old definition of a virus disease is true only in part.