Aetiology according to severity score (CURB-65) (number of patients with known aetiology in specific risk class)
| Aetiology | CURB-65 0–1 (n=741) | CURB-65 2 (n=414) | CURB-65 3–5 (n=308) | Total (n=1463) | p Value |
| Streptococcus pneumoniae | 291 (39) | 180 (43) | 142 (46) | 613 (42) | 0.092 |
| Haemophilus influenzae | 32 (4) | 23 (6) | 15 (5) | 70 (5) | 0.638 |
| Moraxella catarrhalis | 1 (0.1) | 1 (0.2) | 3 (1) | 5 (0.3) | 0.097 |
| Atypical bacteria* † ‡ | 182 (25) | 58 (14) | 23 (7) | 263 (18) | <0.001 |
| Legionella pneumophila | 65 (9) | 36 (9) | 17 (6) | 118 (8) | 0.182 |
| Mycoplasma pneumoniae* † | 55 (7) | 7 (2) | 3 (1) | 65 (4) | <0.001 |
| Chlamydophila pneumoniae† | 36 (5) | 12 (3) | 2 (1) | 50 (3) | 0.002 |
| Coxiella burnetii* † | 26 (4) | 3 (1) | 1 (0.3) | 30 (2) | <0.001 |
| Virus | 81 (11) | 43 (10) | 24 (8) | 148 (10) | 0.301 |
| Staphylococcus aureus | 8 (1) | 11 (3) | 6 (2) | 25 (2) | 0.131 |
| MS* † | 2 (0.3) | 7 (2) | 5 (2) | 14 (1) | 0.024 |
| MR | 6 (1) | 4 (1) | 1 (0.3) | 11 (1) | 0.594 |
| GNEB† ‡ | 10 (1) | 4 (1) | 13 (4) | 27 (2) | 0.002 |
| Pseudomonas aeruginosa* † | 11 (1) | 18 (4) | 21 (7) | 50 (3) | <0.001 |
| Mixed | 101 (14) | 61 (15) | 46 (15) | 208 (14) | 0.806 |
| Others | 24 (3) | 15 (4) | 15 (5) | 54 (4) | 0.441 |