Respiratory disease is one of the most frequently encountered pathological conditions in swine. The history of respiratory conditions in swine is indeed mystifying. At one time it was presumed that the majority of chronic pneumonias were associated with a virus causing enzootic pneumonia or with lung worms. it now seems probable that there are many primary causal agents.
"Viral Causes of Respiratory Diseases in Swine,"
Iowa State University Veterinarian: Vol. 26
, Article 8.
Available at: https://lib.dr.iastate.edu/iowastate_veterinarian/vol26/iss4/8