It may occur in previously normal lungs or be superimposed on underlying bronchitis or other respiratory disease, e. g. bronchiectasis or carcinoma.

• 可以发生在原来正常的肺，亦可在原有支气管炎或其他呼吸道疾病(例如支气管扩张症或肺癌)的基础上再发生本病。