Top 3 Causes of Pneumonia
The three most common causes of community-acquired pneumonia in the general population are: (1) Streptococcus pneumoniae, (2) respiratory viruses (including influenza, RSV, and others), and (3) Mycoplasma pneumoniae.
Primary Bacterial Pathogen
Streptococcus pneumoniae remains the most frequently isolated bacterial pathogen across all settings of community-acquired pneumonia, consistently identified in first place among extracellular bacteria 1.
In hospitalized patients with identified pathogens, S. pneumoniae accounts for approximately 15% of cases when a causative organism is determined, though it represents a much higher proportion when considering only bacterial causes 2.
The IDSA/ATS guidelines explicitly state that S. pneumoniae is the most frequently isolated pathogen in community-acquired pneumonia, occurring across all severity levels from outpatient to ICU settings 1.
Among proven bacterial pneumonias, S. pneumoniae was found in 63% of cases in classic bacteriologic studies 3.
Viral Pathogens
Respiratory viruses are involved in up to 60% of community-acquired lower respiratory tract infections and approximately 30% of community-acquired pneumonia cases 1.
Up to 40% of hospitalized patients with an identified etiology have viruses as the likely cause of pneumonia 2.
Influenza remains the predominant viral cause in adults, with other common viruses including RSV, adenovirus, parainfluenza virus, and human metapneumovirus 1.
Studies including outpatients find viral pneumonia rates as high as 36%, with viruses being the most common etiological agents after S. pneumoniae 1.
Atypical Bacterial Pathogens
Among intracellular bacteria, Mycoplasma pneumoniae is the most common atypical pathogen, followed in frequency by Legionella and Chlamydia species 1.
M. pneumoniae accounts for 4-39% of community-acquired pneumonia cases in various studies, with particularly high rates during epidemic years 1.
In outpatient pneumonia studies, M. pneumoniae was identified in 15-37% of cases, making it a leading cause especially among ambulatory patients 1.
The clinical success rate for levofloxacin in treating atypical pneumonia due to M. pneumoniae was 96%, reflecting its importance as a recognized pathogen 4.
Additional Context on Other Pathogens
Haemophilus influenzae (nontypeable) ranks as the second most common bacterial pathogen after S. pneumoniae, particularly in patients with underlying bronchopulmonary disease 1.
Legionella species, while less common overall (1-15% range), become more frequently encountered in severe pneumonia requiring ICU admission 1.
Mixed infections with two or more pathogens occur in 35% of patients with determined etiology, most commonly S. pneumoniae together with a respiratory virus 1.
Important Clinical Caveat
Only 38% of patients hospitalized with community-acquired pneumonia have a pathogen identified, meaning the majority of cases are treated empirically without microbiologic confirmation 2.
The low yield of bacterial identification in recent studies reflects failure to obtain valid sputum samples before antibiotic administration, rather than absence of bacterial infection 5.
All patients with community-acquired pneumonia should be tested for COVID-19 and influenza when these viruses are common in the community, as their diagnosis affects treatment and infection prevention strategies 2.