What is pneumonia?

What is Pneumonia

Pneumonia is an acute infection of the lung parenchyma (alveoli and distal airways) characterized by respiratory symptoms, physical examination findings, and radiographic evidence of pulmonary infiltrates. 1, 2

Core Definition and Clinical Characteristics

Pneumonia represents an acute pulmonary infection that manifests through a combination of clinical and radiographic findings:

• Clinical presentation includes at least two of the following: fever or hypothermia, rigors, sweats, new cough with or without sputum production, chest discomfort, or dyspnea 1
• Physical examination reveals abnormal respiratory sounds, altered breath sounds, localized rales, or crackles on auscultation 1, 2
• Radiographic confirmation with pulmonary infiltrates on chest imaging is essential for definitive diagnosis 1, 2
• Patients commonly present with nonspecific symptoms including fatigue, myalgias, abdominal pain, anorexia, and headache 1

Classification Systems

By Acquisition Setting

Community-acquired pneumonia (CAP) occurs in previously healthy individuals with infection acquired outside the hospital, defined as symptoms developing in patients not hospitalized or residing in long-term care facilities for >14 days before symptom onset 1, 2

Hospital-acquired (nosocomial) pneumonia develops after 48 hours of hospitalization and was not present at admission, with a distinctly different pathogen spectrum dominated by gram-negative bacilli 1, 2

Ventilator-associated pneumonia is a nosocomial subtype affecting mechanically ventilated patients, occurring in up to 12% of ventilated children 2

By Severity in Pediatric Populations

The WHO classification for resource-limited settings defines pneumonia by age-adjusted tachypnea (age 2-11 months: ≥50/min; 1-5 years: ≥40/min; ≥5 years: >20/min) plus cough or difficulty breathing 1

Severe pneumonia includes lower chest indrawing, nasal flaring, or grunting in addition to respiratory symptoms 1

Very severe pneumonia presents with cyanosis, severe respiratory distress, inability to drink or vomiting everything, or altered consciousness/convulsions 1

By Anatomic Pattern

Simple pneumonia encompasses bronchopneumonia (primary airway and interstitial involvement) or lobar pneumonia involving a single lobe 1

Complicated pneumonia includes parapneumonic effusions, multilobar disease, abscesses, cavities, necrotizing pneumonia, empyema, pneumothorax, bronchopleural fistula, or bacteremic disease with other infection sites 1

Causative Pathogens

The microbial etiology varies significantly by age, setting, and host factors:

• Streptococcus pneumoniae remains the predominant bacterial cause in adults, accounting for 50-90% of bacterial CAP cases 1, 2
• Viral pathogens (respiratory syncytial virus, influenza, adenovirus) predominate in infants, toddlers, and preschool children 2
• Mycoplasma pneumoniae causes atypical pneumonia with slow progression, accounting for 8-16% of hospitalizations in school-aged children and young adolescents 2
• Hospital-acquired pneumonia is predominantly caused by gram-negative bacilli and differs substantially from community pathogens 2
• A wide variety of bacteria, viruses, and fungi can cause pneumonia depending on host immune status and epidemiologic exposures 2, 3

Epidemiologic Burden

Pneumonia represents a major global health problem with substantial morbidity and mortality:

• Sixth leading cause of death and the number one cause of death from infectious diseases in the United States 2
• Accounts for 16% of all deaths in children under 5 years old worldwide 2
• Approximately 2-3 million cases annually in the United States result in ~10 million physician visits, 500,000 hospitalizations, and 45,000 deaths 1
• Hospitalization incidence is 258 per 100,000 population overall and 962 per 100,000 in those aged >65 years 1
• Mortality averages 14% among hospitalized patients but is <1% for outpatients 1
• The incidence peaks during winter months 1

Pathophysiology

Disease development depends primarily on the host immune response rather than pathogen characteristics alone 3

Susceptible populations include children <5 years of age and older adults with chronic conditions 3

The inflammatory response during pneumonia can persist and remain dysregulated post-infection, contributing to accelerated health decline and increased cardiovascular disease risk in survivors 4

Important Clinical Considerations

Do not rely on chest radiography alone to distinguish viral from bacterial pneumonia or identify specific bacterial pathogens—clinical context and risk factors are essential 2

Up to 20% of patients with non-resolving pneumonia have diseases other than CAP, including malignancy, pulmonary embolism, inflammatory conditions, and empyema or abscess formation 5

Delayed or inadequate antimicrobial therapy leads to poor outcomes, making pathogen identification and appropriate treatment selection crucial 3