Mycoplasmal hyopneumonia is a chronic, typically clinically mild, infectious pneumonia of pigs. 

The disease is characterized by a persistent dry cough, impaired growth, occasional flares of overt respiratory distress, and a high incidence of lung lesions in slaughtered pigs. 

It tends to become endemic in infected herds and occurs worldwide.

Diagnostic monitoring of herds presumably free of mycoplasmal pneumonia can be difficult. It is hypothesized that the organism has not been successfully eliminated but rather coexisted within the population at an undetectable level for extended periods. However, achieving a low prevalence, regardless of lack of pathogen elimination, has proven to be a significant production advantage in large swine units.