Pneumonia is defined as acute infection of the lung parenchyma, and when that infection is acquired outside of the hospital setting, it is referred to as community-acquired pneumonia (CAP). See hospital-acquired pneumonia for discussion of pneumonia acquired in the hospital setting.

CAP remains a leading cause of morbidity and mortality, accounting for up to 2 million deaths per year in children younger than 5 years old worldwide. Clinical signs of an infectious lung process include fever, production of sputum (which is often purulent), leukocytosis, and hypoxia, but presentations can vary and the provider's index of suspicion for pneumonia should be high when evaluating any respiratory illness. No symptom or set of symptoms is adequate for the diagnosis of pneumonia without chest imaging.

In the United States, CAP accounts for over 4.5 million emergency room and outpatient visits annually, accounting for 0.4% of encounters. It is the most common infectious cause of death and one of the most common cause for hospitalization, with 1.5 million CAP hospitalizations annually. Globally and locally, the true incidence of CAP is difficult to determine accurately due to differences in reporting and case definition. Incidence also varies significantly by geographic location, study population, and season, with rates almost doubling in winter months.

The microbiologic etiology of CAP has evolved over time. Historically, in the pre-antibiotic era, almost all cases (95%) were caused by Streptococcus pneumoniae. Since the advent of antibiotics as well as the widespread use of pneumococcal conjugate vaccines in children and pneumococcal polysaccharide vaccines in adults, the rate of S pneumoniae pneumonia in the United States has dropped to 10%-15% of cases. Other bacterial etiologies include Haemophilus influenzae, Moraxella catarrhalis, Staphylococcus aureus, group A Streptococcus spp, and Pseudomonas aeruginosa, as well as other gram-negative bacilli. Etiologies categorized as "atypical" include Mycoplasma pneumoniae, Chlamydophila pneumoniae, and Legionella pneumophilia, which have a wide variability in reported incidence in part due to their inability to grow on standard culture media and variable diagnostic techniques. Of note, respiratory viruses have been identified in up to a third of CAP cases when studied using molecular techniques. The exact impact of this finding – whether viral infection can be the primary pathogen, a cofactor for infection, or a trigger for a dysregulated immune response – is unknown and warrants further study.

Risk factors for CAP include under-vaccination with pneumococcal virus (PCV), Haemophilus influenzae type b (Hib), diphtheria, tetanus, and pertussis (ie, DTap, Tdap, DT) and influenza vaccines; age younger than 6 months; prematurity; preexisting lung or heart disease; cerebral palsy; seizures or neurologic impairment; and weakened immune system. Preceding viral infection increases the risk for bacterial superinfection, classically seen with respect to influenza and subsequent risk for pneumonia attributed to S aureus.

CAP is classified as severe if it meets 1 major criterium or 3 minor criteria.

Major criteria:

• Septic shock with need of vasopressors
• Respiratory failure necessitating mechanical ventilation

Minor criteria:

• Respiratory rate ≥ 30 breaths per minute
• PaO2/FiO2 ≤ 250 mmHg
• Multilobar infiltrates
• Altered mental status
• Uremia (blood urea nitrogen [BUN] ≥ 20 mg/dl)
• Leukopenia (< 4000 WBC)
• Thrombocytopenia (platelets < 100 000)
• Hypothermia (< 36°C / 96.8°F)
• Hypotension requiring aggressive fluid resuscitation

Although the majority of patients with CAP recover, it remains among the leading causes of death worldwide, either due to overwhelming infection with sepsis or respiratory failure or due to decompensation of other comorbid complications (cardiovascular events, etc). Mortality can reach up to 20%-25% in severe CAP, although it is < 1% in ambulatory patients.

Note: To view x-rays and other imagery, see individual diagnoses.

Related topics: Klebsiella pneumonia, herpes simplex pneumonia, varicella pneumonia