| Streptococcus pneumoniae (pneumococcus). Gram-positive diplococci. | Chronic cardiopulmonary disease; follows upper respiratory tract infection | Bacteremia, meningitis, endocarditis, pericarditis, empyema |
| Haemophilus influenzae. Pleomorphic gram-negative coccobacilli. | Chronic cardiopulmonary disease; follows upper respiratory tract infection | Empyema, endocarditis |
| Staphylococcus aureus. Plump gram-positive cocci in clumps. | Residence in long-term care facility, hospital-associated, influenza epidemics, cystic fibrosis, bronchiectasis, injection drug use | Empyema, cavitation |
| Klebsiella pneumoniae. Plump gram-negative encapsulated rods. | Alcohol abuse, diabetes mellitus; hospital-associated | Cavitation, empyema |
| Escherichia coli. Gram-negative rods. | Hospital-associated; rarely, community-acquired | Empyema |
| Pseudomonas aeruginosa. Gram-negative rods. | Hospital-associated; cystic fibrosis, bronchiectasis | Cavitation |
| Anaerobes. Mixed flora. | Aspiration, poor dental hygiene | Necrotizing pneumonia, abscess, empyema |
| Mycoplasma pneumoniae. PMNs and monocytes; no bacteria. | Young adults; summer and fall | Skin rashes, bullous myringitis; hemolytic anemia |
| Legionella species. Few PMNs; no bacteria. | Summer and fall; exposure to contaminated construction site, water source, air conditioner; community-acquired or hospital-associated | Empyema, cavitation, endocarditis, pericarditis |
| Chlamydophila pneumoniae. Nonspecific. | Clinically similar to M pneumoniae, but prodromal symptoms last longer (up to 2 weeks); sore throat with hoarseness common; mild pneumonia in teenagers and young adults | Reinfection in older adults with underlying COPD or heart failure may be severe or even fatal |
| Moraxella catarrhalis. Gram-negative diplococci. | Preexisting lung disease; elderly patients; corticosteroid or immunosuppressive therapy | Rarely, pleural effusions and bacteremia |
| Pneumocystis jirovecii. Nonspecific. | AIDS, immunosuppressive or cytotoxic drug therapy, cancer | Pneumothorax, respiratory failure, ARDS, death |
| SARS-CoV-2. Nonspecific. | Pandemic. Milder pneumonia (teenagers, young adults); more severe pneumonia (elderly, immunocompromised, multiple comorbidly ill adults) | Respiratory failure, ARDS, death |