Proposed etiologies of IAD include allergic airway disease, recurrent pulmonary stress, deep inhalation of dust, atmospheric pollutants, and/or persistent respiratory viral infections, most notably EHV-2. IAD often develops after an overt viral respiratory infection and may result from inability of the immune system to fully eliminate viruses or bacteria from small airways. Streptococcus pneumoniae has been isolated from horses with IAD; however, its role in the pathophysiology is unclear, because this population of horses is largely unresponsive (or transiently responsive) to antibiotic therapy.