Lung transplant patients are susceptible to opportunistic infections, and more than 50% experience one or more episodes of infection resulting in increased morbidity and occasional fatality. Immunosuppressive therapy, loss of cough reflex, and ischemia of the anastomotic site all increase the risk of infection in the transplant patient, with infection occurring either in the transplanted lungs or, in the case of a single transplant, the infection may first present in the native lung.

Most infections are bacterial in nature, with gram-negative organisms, especially Pseudomonas, predominating, and with diagnosis made by culture rather than based on tissue histology. In bacterial infections, tissue sections of the transplant lung show variable degrees of lung parenchyma involved by bronchopneumonia, bronchitis, diffuse alveolar damage, and occasionally abscess formation.

More often histologic examination of transplant lung biopsy tissue confirms viral and fungal infection with the addition of special stains (immunohistochemical, GMS). CMV (cytomegalovirus) is the most frequent viral infection, with HSV (herpes simplex virus), RSV (respiratory syncytial virus), and adenovirus occurring less commonly. The viral pneumonias can be seen as interstitial pneumonias, diffuse alveolar damage, ulceration, and inflammation of trachea and bronchioles.