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 1.27 million deaths per year in children younger than 5 years worldwide. Clinical signs 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; however, chest imaging is not needed in all pediatric patients to establish the diagnosis. If the patient is otherwise healthy with nonsevere symptoms, a history and physical examination are adequate to make this diagnosis.

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 causes of hospitalization, with 15.7 hospitalizations per 10 000 children. Worldwide and locally, the true incidence of CAP is difficult to determine accurately due to differences in reporting and case definition. Incidence 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 preantibiotic era, amost all cases (95%) were caused by Streptococcus pneumoniae. Since the advent of the widespread use of pneumococcal conjugate vaccines and the Haemophilus influenza type B vaccine in children, the rate of bacterial pneumonia in the United States has dropped, with viruses now accounting for the majority of radiographically confirmed CAP in children (> 90%). Recent evidence (albeit pre-COVID-19 pandemic) supports that, in pediatric CAP patients younger than 5 years, the most common pathogen isolated is respiratory syncytial virus (RSV), a viral pathogen, followed by streptococcal pneumonia. In those 5 years and older, Mycoplasma pneumonia was more prevalent than in the younger age group.

Risk factors for CAP include undervaccination 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; pre-existing 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 in respect to influenza and the subsequent risk for pneumonia attributed to S aureus.

Unlike adult CAP, there are no clearly agreed upon criteria for severe CAP in pediatrics. The Pediatric Infectious Diseases Society (PIDS) / Infectious Diseases Society of America (IDSA) guidelines from 2011 describe moderate-to-severe CAP as marked by respiratory distress and hypoxemia (sustained oxygen saturation [SpO2] < 90%) and requiring hospitalization.

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 less than 1% in ambulatory patients.