What causes pneumonia?

What Causes Pneumonia

Pneumonia is caused by a wide variety of infectious pathogens including bacteria, viruses, and fungi, with the specific causative organism varying significantly based on patient age, acquisition setting (community vs. hospital), and immune status. 1

Bacterial Causes

Most Common Bacterial Pathogen

• Streptococcus pneumoniae (pneumococcus) is the predominant bacterial cause of community-acquired pneumonia across all adult age groups, accounting for 50-90% of bacterial pneumonia cases. 2, 1
• In outpatient settings, pneumococcus is identified in 9-20% of all pneumonia episodes when sputum culture is used. 2, 3
• Pneumococcal pneumonia occurs more frequently in elderly patients, alcoholics, and those with chronic cardiovascular disease, chronic obstructive pulmonary disease, immunoglobulin deficiency, hematologic malignancy, and HIV infection. 2

Other Important Bacterial Pathogens

• Haemophilus influenzae causes 3-10% of community-acquired pneumonia episodes, particularly in smokers and patients with chronic obstructive pulmonary disease. 2, 3
• Staphylococcus aureus accounts for up to 10% of cases and is especially common following influenza infection. 2
• Gram-negative enteric bacilli (Klebsiella pneumoniae, Escherichia coli, Enterobacter species) cause pneumonia primarily in patients with specific risk factors and account for a minority of community-acquired cases but are more prevalent in hospital-acquired pneumonia. 2
• Legionella species cause 0.7-13% of outpatient pneumonia and 5-20% of hospitalized cases, with higher rates in organ transplant recipients, patients with renal failure, chronic lung disease, and smokers. 2

Atypical Bacterial Pathogens

• Mycoplasma pneumoniae is the most common atypical pathogen when serologic testing is performed, accounting for 13-37% of outpatient episodes and 8-16% of hospitalizations in school-aged children and young adolescents. 2, 3
• Mycoplasma causes atypical pneumonia characterized by slow progression, malaise, and low-grade fever. 2, 1
• Chlamydia pneumoniae has been reported in up to 17% of outpatients with community-acquired pneumonia. 2, 3
• These atypical pathogens are found across all age groups, not just in young healthy patients, and are often part of mixed infections. 2

Viral Causes

Pediatric Populations

• In infants, toddlers, and preschool children, viruses are the most common pathogens overall, with respiratory syncytial virus being the usual culprit. 2, 1, 4
• Respiratory viruses account for the majority of pneumonia in children under 5 years of age. 2

Adult Populations

• Respiratory viruses are detected in 10-36% of immunocompetent adults hospitalized with community-acquired pneumonia. 2, 3
• Influenza virus is the most important viral cause in adults, particularly in elderly patients and those with chronic obstructive lung disease. 2, 5
• Respiratory syncytial virus (RSV) causes significant morbidity in elderly patients and those with cardiopulmonary disease. 2
• Other viral pathogens include parainfluenza virus, adenovirus, metapneumovirus, varicella zoster virus, and herpes simplex virus. 2, 5

Fungal Causes

• Endemic fungi (Histoplasma capsulatum, Coccidioides immitis, Blastomyces dermatitidis) should be considered in patients with travel history to endemic areas. 3, 5
• Pneumocystis jirovecii is an important cause in HIV-infected patients and other immunosuppressed individuals. 1, 3
• Opportunistic fungi (Candida, Aspergillus, Phycomycetes) cause pneumonia primarily in compromised hosts. 5

Age-Specific Patterns

Children

• Viral pathogens predominate in children under 5 years, with respiratory syncytial virus most common. 2, 4
• After 3 months of age, Streptococcus pneumoniae becomes the most common bacterial pathogen, though this is changing with increased pneumococcal vaccination. 2, 4
• In school-aged children and adolescents, bacterial pneumonia becomes more common, with S. pneumoniae and M. pneumoniae as leading causes. 2

Adults

• S. pneumoniae remains the predominant pathogen across all adult age groups. 2, 1
• Atypical pathogens and viruses contribute significantly, with rates varying by diagnostic methods used. 2

Hospital-Acquired Pneumonia

• Hospital-acquired pneumonia develops after 48 hours of hospitalization and has a distinctly different pathogen spectrum than community-acquired pneumonia. 2, 1
• Gram-negative bacilli predominate, including Pseudomonas aeruginosa (16.9%), Klebsiella species (11.6%), Enterobacter species (9.4%), and Escherichia coli. 2
• Staphylococcus aureus accounts for 12.9% of nosocomial pneumonia cases. 2
• Ventilator-associated pneumonia affects up to 12% of mechanically ventilated children. 2, 1

Mixed Infections

• Mixed infections involving both bacterial and atypical pathogens, or bacterial and viral pathogens, occur in 3-14% of cases. 2, 3
• Some studies suggest coinfection may lead to more complicated courses, though this remains controversial. 2

Diagnostic Limitations

Despite current diagnostic techniques, no pathogen is identified in 40-70% of community-acquired pneumonia cases. 2, 3 This reflects prior antibiotic treatment, presence of unusual pathogens, viral infections, noninfectious mimics, and limitations of current diagnostic methods. 2