Pathogenesis of lower respiratory tract infections due to Chlamydia, Mycoplasma, Legionella and viruses.

Acute infection of the lower respiratory tract comprises bronchitis, bronchiolitis, and pneumonia. From a clinical point of view it may be diYcult to distinguish these disease entities and one infection may progress into another. The most common pathogens causing these infections are the primary respiratory viruses (respiratory syncytial virus (RSV), influenza virus, adenoviruses, parainfluenza virus, and rhinovirus ), Mycoplasma pneumoniae, and Chlamydia species. Legionella may cause pneumonia and non-pneumonic upper respiratory tract infection and approximately 85% of c ases are caused by L pneumophila. Long lasting sequelae such as bronchiectasis, lung fibrosis, and decreased lung function are seen after lower respiratory tract infections and it has been debated whether respiratory tract infection can cause chronic bronchial asthma. 11–13