| Aetiology | Cases n (%)# | Documented organisms¶ (n) |
| Total RD | 163 (35.4) | |
| Bilateral lesions | 140 (30.4) | |
| AE | 90 (19.5) | |
| Definite | 57 (12.4) | |
| Suspected | 33 (7.2) | |
| Infection | 37 (8.0) | |
| Definite | 21 (4.6) | |
| Bacterial | 9 (2.0) | Streptococcus pneumoniae (2)MRSA (1)Haemophilus influenzae (4)Legionella spp. (1)Klebsiella pneumoniae (1) |
| Viral | 7 (1.5) | CMV (7; 2 mixed infections with RSV or Pneumocystis jiroveci)Influenza virus (1)RSV (1) |
| Fungal | 2 (0.4) | Candida spp. (1)Aspergillus spp. (1) |
| Parasitic | 2 (0.4) | Pneumocystis jiroveci (2) |
| Mycobacterial | 1 (0.2) | Mycobacterium tuberculosis (1) |
| Suspected+ | 16 (3.5) | |
| Heart failure | 5 (1.1) | |
| PTE | 2 (0.4) | |
| AEP | 1 (0.2) | |
| Uncertain§ | 5 (1.1) | |
| Focal lesion | 23 (5.0) | |
| Pneumothorax | 9 (2.0) | |
| Infection | 14 (3.0) | Klebsiella pneumoniae (1)Klebsiella oxytoca (1)Streptococcus pneumoniae (1) |
AE: acute exacerbation; PTE: pulmonary thromboembolism; AEP: acute eosinophilic pneumonia; MRSA: methicillin-resistant Staphylococcus aureus; CMV: cytomegalovirus; RSV: respiratory syncytial virus. #: percentage of total subjects (n = 461); ¶: identified by culture, antigen test (for S. pneumoniae and Legionella), or direct fluorescence monoclonal antibody stain (for P. jiroveci); +: cases in which no organism was identified, but infection was clinically suspected due to symptoms such as grossly purulent sputum and rapid resolution of symptoms in response to antibiotics alone; §: cases in which no organism was identified, but the criteria for AE were not completely satisfied due to incomplete study.