||Diagnosis of a virus which is always virulent, e.g. FMDV, is
definitive. Diagnosis of a virus which can be avirulent, e.g. Newcastle disease, is more
difficult because the virus may not have caused the disease but be a subclinical infection.
Diagnosis of a new virus from a new disease is the most difficult because the virus must
be shown to cause disease by fulfilling Koch's 3 postulates: isolating the virus in pure
culture; using the pure virus to reproduce the new disease in a healthy host, e.g.PRRS
into a healthy piglet; recovering the virus from the lesions of the experimental animal.