Iowa State University Veterinarian Records, RS 22/6/0/9, University Archives, Special Collections Department, Iowa State University, http://www.add.lib.iastate.edu/spcl/arch/rgrp/22-6-0-9.html
Recognition of a new disease in swine may occur because of one of two reasons. The first is occasioned by the recognition of an existing but previously overlooked specific disease agent and its attendant clinical syndrome while the second is due to the introduction of a hitherto absent pathogen into an area, region, or country. In general, it requires some interval of time to be reasonably certain which of these two situations is applicable to a newly recognized clinical problem.
Switzer, W. P.
"Inclusion Body Rhinitis,"
Iowa State University Veterinarian: Vol. 25
, Article 8.
Available at: https://lib.dr.iastate.edu/iowastate_veterinarian/vol25/iss3/8