Definition and epidemiology
Pneumonia is defined as an acute infection of the pulmonary parenchyma associated with clinical symptoms (cough, fever, sputum production, dyspnea) and radiographic evidence of infiltrate. It is classified by the setting of acquisition into community-acquired pneumonia (CAP), hospital-acquired pneumonia (HAP), ventilator-associated pneumonia (VAP), and healthcare-associated pneumonia.
Pneumonia is a leading cause of morbidity and mortality worldwide. Globally, pneumonia causes approximately 2.5 million deaths annually, with the highest burden in children under 5 years and adults over 65 years. In high-income countries, the annual incidence of CAP is approximately 5-11 per 1,000 adults, increasing substantially with age. Pneumonia is the most common infectious cause of death in developed nations and remains a top cause of hospitalization.
Risk is higher in winter months due to increased viral respiratory infections. The elderly, immunocompromised patients, those with chronic lung disease, and smokers face disproportionate risk. Hospital-acquired pneumonia develops 48 hours or more after admission and carries higher mortality due to multidrug-resistant organisms.
- Streptococcus pneumoniae is the most common cause of community-acquired pneumonia in all age groups.
- Rusty or blood-tinged sputum is classically associated with pneumococcal pneumonia.
- Klebsiella pneumoniae causes currant-jelly sputum and occurs in alcoholics; may produce bulging fissure sign.
- Mycoplasma pneumoniae causes walking pneumonia in young adults; associated with cold agglutinins, hemolytic anemia, and erythema multiforme.
- Legionella pneumonia is associated with hyponatremia, diarrhea, confusion, and exposure to water systems; diagnosed by urinary antigen.
- Staphylococcus aureus pneumonia often follows influenza infection and causes cavitation and pneumatoceles.
- Air bronchograms on chest X-ray indicate alveolar consolidation with patent bronchi—classic finding in pneumonia.
- CURB-65 score guides disposition: 0-1 outpatient, 2 consider admission, ≥3 hospitalize and consider ICU.
Etiology and risk factors
Streptococcus pneumoniae (pneumococcus) is the most common bacterial cause of community-acquired pneumonia in all age groups, accounting for approximately 30-50% of cases requiring hospitalization. Other common bacterial pathogens include Haemophilus influenzae, Staphylococcus aureus (including MRSA, especially post-influenza), Moraxella catarrhalis, and gram-negative bacilli (Klebsiella pneumoniae in alcoholics, associated with currant-jelly sputum).
Atypical pathogens include Mycoplasma pneumoniae (young adults, walking pneumonia, extrapulmonary manifestations like hemolytic anemia and erythema multiforme), Chlamydophila pneumoniae (mild, community outbreaks), and Legionella pneumophila (water sources, air conditioning systems, hyponatremia, diarrhea, confusion). Viral causes include influenza, RSV, SARS-CoV-2, adenovirus, and parainfluenza.
Hospital-acquired pneumonia is frequently caused by Pseudomonas aeruginosa, MRSA, Enterobacteriaceae, and Acinetobacter. Aspiration pneumonia typically involves oral anaerobes (Bacteroides, Peptostreptococcus, Fusobacterium) and occurs in dependent lung segments.
Risk factors
- Age >65 years or <2 years
- Chronic diseases: COPD, heart failure, diabetes, chronic kidney disease
- Immunosuppression: HIV/AIDS, malignancy, corticosteroids, chemotherapy
- Smoking and alcohol abuse
- Aspiration risk: altered mental status, dysphagia, neurological disorders
- Recent viral respiratory infection (influenza)
- Crowded living conditions, nursing home residence
- Hospitalization, mechanical ventilation
Pathophysiology
Pneumonia develops when microbial pathogens reach the alveolar space and overcome host defenses. The usual route is microaspiration of oropharyngeal secretions, though inhalation of infectious aerosols, hematogenous spread, and direct extension from contiguous sites also occur.
Once in the alveoli, bacteria proliferate and trigger an acute inflammatory response. Neutrophils and inflammatory mediators flood the alveolar spaces, causing consolidation—the replacement of air with inflammatory exudate, bacteria, and cellular debris. This creates the characteristic dense opacity on chest X-ray. Capillary leakage and edema further impair gas exchange, leading to hypoxemia.
The inflammatory process follows classic stages in lobar pneumonia: congestion (vascular engorgement, bacterial proliferation), red hepatization (alveoli filled with red blood cells, neutrophils, fibrin), gray hepatization (red cell disintegration, persistence of fibrin and inflammatory cells), and resolution (enzymatic degradation of exudate, restoration of normal architecture). Bronchopneumonia involves patchy, multifocal consolidation centered on bronchioles rather than lobar distribution.
Impaired gas exchange results from ventilation-perfusion mismatch and intrapulmonary shunting. Systemic manifestations (fever, tachycardia, leukocytosis) reflect cytokine release. Severe cases may progress to sepsis, acute respiratory distress syndrome (ARDS), and multiorgan failure.
Clinical features
The classic presentation includes acute onset of fever, productive cough with purulent sputum, pleuritic chest pain, and dyspnea. However, clinical presentation varies by pathogen, patient age, and immune status.
Symptoms
- Cough: typically productive with yellow, green, or rust-colored sputum; may be dry initially
- Fever and chills: abrupt high fever with rigors suggests bacterial pneumonia
- Dyspnea: increased work of breathing, tachypnea
- Pleuritic chest pain: sharp, worsens with inspiration or cough
- Fatigue, myalgias, headache: common with viral and atypical pathogens
- Gastrointestinal symptoms: nausea, vomiting, diarrhea (especially Legionella)
Physical examination findings
- Vital signs: fever (>38°C), tachycardia, tachypnea, hypoxemia
- Inspection: use of accessory muscles, cyanosis in severe cases
- Palpation: increased tactile fremitus over consolidated areas
- Percussion: dullness over affected lobe
- Auscultation: crackles (rales), bronchial breath sounds, egophony (E-to-A change), whispered pectoriloquy, decreased breath sounds if effusion present
Special populations
Elderly patients often present atypically with confusion, falls, failure to thrive, or worsening of chronic conditions without prominent respiratory symptoms or fever. Immunocompromised patients may have blunted inflammatory response with minimal signs despite severe disease. Atypical pneumonia (Mycoplasma, Chlamydophila) presents insidiously with low-grade fever, dry cough, and prominent extrapulmonary symptoms; examination findings may be minimal.
Diagnosis
Diagnosis requires clinical suspicion supported by radiographic evidence of infiltrate. The combination of symptoms, physical examination, and chest imaging establishes the diagnosis.
Chest X-ray
Chest X-ray is the standard initial imaging test and demonstrates infiltrates, consolidation, or interstitial patterns. Lobar consolidation with air bronchograms is classic for bacterial pneumonia. Patchy, bilateral interstitial infiltrates suggest atypical or viral pneumonia. Pneumococcal pneumonia often shows dense lobar involvement, while Staphylococcus may produce cavitation and pneumatoceles. Klebsiella classically causes bulging fissure sign. The presence of pleural effusion should prompt consideration of parapneumonic effusion or empyema.
Chest X-ray may be normal early in disease or in dehydrated patients. If clinical suspicion is high and initial X-ray is negative, repeat imaging after hydration or obtain CT chest.
Laboratory investigations
- Complete blood count: leukocytosis with left shift (bacterial); normal or leukopenia (viral, atypical)
- Inflammatory markers: elevated CRP, ESR, procalcitonin (procalcitonin >0.25 ng/mL suggests bacterial infection)
- Arterial blood gas: if hypoxemia suspected; assess oxygenation and acid-base status
- Blood cultures: recommended for hospitalized patients before antibiotics; positive in 10-20% of CAP
- Sputum Gram stain and culture: if able to produce quality sample; guides therapy in HAP
- Urinary antigen tests: for Streptococcus pneumoniae and Legionella pneumophila (serogroup 1); rapid, remains positive after antibiotic initiation
Additional testing
Thoracentesis is indicated if pleural effusion is >1 cm on lateral decubitus film; analyze fluid for cell count, Gram stain, culture, pH, glucose, LDH, protein. CT chest is more sensitive than X-ray and useful when diagnosis is unclear, complications are suspected, or patient is not responding to therapy. PCR testing for influenza, SARS-CoV-2, and other respiratory viruses guides antiviral therapy and infection control.
Severity assessment
Severity scores guide disposition decisions. CURB-65 assigns one point each for: Confusion, Urea >7 mmol/L (BUN >20 mg/dL), Respiratory rate ≥30, Blood pressure <90/60 mmHg, age ≥65. Score 0-1: outpatient; 2: consider admission; ≥3: hospitalize, consider ICU. PSI (Pneumonia Severity Index) is more complex but highly validated for mortality prediction.
Differential diagnosis
The differential diagnosis includes other causes of acute respiratory symptoms and radiographic infiltrates:
- Acute bronchitis: cough without infiltrate on imaging
- Congestive heart failure: bilateral infiltrates, cardiomegaly, Kerley B lines, elevated BNP
- Pulmonary embolism: dyspnea, pleuritic pain, may have infiltrate (Hampton's hump); risk factors for thromboembolism
- Lung cancer: chronic symptoms, mass lesion, obstructive pneumonia
- Pulmonary tuberculosis: chronic cough, night sweats, weight loss, upper lobe cavitation, risk factors
- Aspiration pneumonitis (chemical): acute respiratory distress after witnessed aspiration; dependent segments
- Acute interstitial lung disease: subacute onset, bilateral infiltrates, connective tissue disease history
- Pulmonary eosinophilia: peripheral eosinophilia, migratory infiltrates
- Pulmonary contusion: trauma history, alveolar hemorrhage
- Cryptogenic organizing pneumonia: peripheral consolidation, responds to corticosteroids
Management
Management involves supportive care, appropriate antimicrobial therapy, and monitoring for complications. The choice of antibiotics depends on the setting (outpatient vs. inpatient), severity, local resistance patterns, and risk factors for specific pathogens.
Outpatient management (community-acquired pneumonia)
Healthy adults with no recent antibiotic use: amoxicillin or doxycycline or macrolide (azithromycin, clarithromycin). If comorbidities (diabetes, heart/lung/liver/kidney disease, immunosuppression) or recent antibiotic use: respiratory fluoroquinolone (levofloxacin, moxifloxacin) or beta-lactam plus macrolide (amoxicillin-clavulanate plus azithromycin).
Inpatient non-ICU management
Beta-lactam (ampicillin-sulbactam, ceftriaxone, cefotaxime) plus macrolide (azithromycin), or respiratory fluoroquinolone monotherapy. The combination provides coverage for typical and atypical pathogens.
Inpatient ICU management (severe CAP)
Beta-lactam (ceftriaxone, cefotaxime, ampicillin-sulbactam) plus azithromycin or plus fluoroquinolone. If MRSA risk (recent influenza, necrotizing pneumonia, prior MRSA, positive nasal screen): add vancomycin or linezolid. If Pseudomonas risk (structural lung disease, recent broad-spectrum antibiotics, recent hospitalization): use antipseudomonal beta-lactam (piperacillin-tazobactam, cefepime, meropenem).
Hospital-acquired and ventilator-associated pneumonia
Empiric therapy must cover resistant gram-negatives and MRSA: antipseudomonal beta-lactam (piperacillin-tazobactam, cefepime, meropenem) plus vancomycin or linezolid. De-escalate based on culture results and clinical response.
Aspiration pneumonia
Ampicillin-sulbactam or clindamycin or piperacillin-tazobactam to cover oral anaerobes.
Duration of therapy
Most uncomplicated CAP: 5-7 days. Adjust based on clinical stability (afebrile for 48-72 hours, improving symptoms, tolerating oral intake, normal vital signs). Longer courses for complicated pneumonia, slow responders, or specific pathogens (Pseudomonas, Staphylococcus).
Supportive care
- Oxygen therapy: maintain SpO2 >90-92%
- Intravenous fluids: for dehydration, sepsis
- Antipyretics and analgesics: acetaminophen for fever and pain
- Chest physiotherapy and incentive spirometry: promote secretion clearance
- Vaccination: pneumococcal and influenza vaccines reduce future risk
Monitoring and follow-up
Reassess clinical response at 48-72 hours. Failure to improve suggests resistant organism, wrong diagnosis, complications (empyema, abscess), or non-infectious etiology. Follow-up chest X-ray at 6-8 weeks is recommended in smokers and adults >50 to exclude underlying malignancy, especially if slow radiographic resolution.
Complications and prognosis
Complications of pneumonia can be pulmonary or extrapulmonary and significantly increase morbidity and mortality.
Pulmonary complications
- Parapneumonic effusion and empyema: pleural fluid accumulation; empyema requires drainage (chest tube, VATS)
- Lung abscess: cavitary lesion with air-fluid level; prolonged antibiotics, possible drainage
- Necrotizing pneumonia: extensive tissue destruction; Staphylococcus aureus, Klebsiella, anaerobes
- Acute respiratory distress syndrome (ARDS): severe hypoxemia, bilateral infiltrates, requires mechanical ventilation
- Respiratory failure: inability to maintain adequate oxygenation or ventilation
- Pneumothorax: especially with necrotizing infection or ventilator use
Extrapulmonary complications
- Sepsis and septic shock: systemic inflammatory response, hypotension, organ dysfunction
- Bacteremia: bloodstream infection, risk of metastatic infection (endocarditis, meningitis, arthritis)
- Multiorgan failure: kidney injury, hepatic dysfunction, coagulopathy
- Cardiovascular events: myocardial infarction, arrhythmias, heart failure exacerbation
Prognosis
Overall mortality from community-acquired pneumonia is approximately 5-15% for hospitalized patients and <1% for outpatients. Mortality increases substantially with age, comorbidities, and severity. ICU admission is required in 10-20% of hospitalized CAP patients, with mortality rates of 20-40% in this group. Hospital-acquired pneumonia has higher mortality (20-50%) due to resistant organisms and underlying illness.
Most patients show clinical improvement within 3-5 days and radiographic resolution within 4-6 weeks. Delayed resolution occurs in elderly patients, those with multilobar involvement, bacteremia, or specific pathogens (Legionella, Staphylococcus). Factors associated with poor prognosis include advanced age, multiple comorbidities, hypoxemia, hypotension, altered mental status, renal dysfunction, and leukopenia.
Prevention strategies include vaccination (pneumococcal conjugate and polysaccharide vaccines, annual influenza vaccine), smoking cessation, hand hygiene, and aspiration precautions in high-risk patients.
CURB-65 severity assessment: Confusion, Urea >7 mmol/L, Respiratory rate ≥30, Blood pressure <90/60, age ≥65 years.
Practice questions
1. A 68-year-old man with COPD presents with 3 days of fever, productive cough with yellow-green sputum, and shortness of breath. Temperature is 38.9°C, respiratory rate 28/min, blood pressure 110/70 mmHg, and oxygen saturation 89% on room air. Physical examination reveals decreased breath sounds and crackles in the right lower lung field. Chest X-ray shows right lower lobe consolidation. Which of the following is the most appropriate initial treatment?
2. A 32-year-old woman presents with 1 week of low-grade fever, nonproductive cough, and fatigue. She reports no significant past medical history and works as a schoolteacher. Temperature is 37.8°C, and vital signs are otherwise normal. Lung examination reveals scattered crackles bilaterally. Chest X-ray shows bilateral diffuse interstitial infiltrates. Laboratory studies show hemoglobin 9.5 g/dL, WBC 11,000/µL, and positive cold agglutinins. Which of the following is the most likely causative organism?
3. A 75-year-old nursing home resident with dementia is brought to the emergency department with altered mental status. Caregivers report he has been less responsive over the past 2 days. He has a history of stroke with dysphagia. Temperature is 38.3°C, blood pressure 95/60 mmHg, respiratory rate 24/min, and oxygen saturation 91% on room air. Examination reveals foul-smelling breath and crackles in the right lower lung. Chest X-ray shows right lower lobe infiltrate with a small pleural effusion. Which of the following organisms is most likely responsible?
Frequently asked questions
What causes pneumonia?
Pneumonia is caused by infection with bacteria, viruses, or fungi. Streptococcus pneumoniae (pneumococcus) is the most common bacterial cause, responsible for approximately 30-50% of community-acquired pneumonia cases. Other common bacterial causes include Haemophilus influenzae, Staphylococcus aureus, Mycoplasma pneumoniae, and Legionella. Viral causes include influenza, RSV, and SARS-CoV-2. Hospital-acquired pneumonia is often caused by multidrug-resistant organisms like Pseudomonas and MRSA. Risk factors include advanced age, chronic diseases, immunosuppression, smoking, and aspiration.
What are the first symptoms of pneumonia?
The first symptoms of pneumonia typically include sudden onset of fever, cough (initially dry, then productive with purulent sputum), and shortness of breath. Many patients also experience pleuritic chest pain that worsens with breathing or coughing, chills with rigors, fatigue, and myalgias. However, presentation varies: elderly patients may present with confusion or falls rather than respiratory symptoms, and atypical pneumonia (Mycoplasma, Chlamydophila) may cause gradual onset of dry cough with low-grade fever. Any combination of acute respiratory symptoms with fever should prompt evaluation for pneumonia.
How is pneumonia diagnosed?
Pneumonia is diagnosed by the combination of clinical symptoms (fever, cough, dyspnea), physical examination findings (crackles, bronchial breath sounds, dullness to percussion), and chest X-ray showing infiltrate or consolidation. Chest X-ray is the key diagnostic test and typically reveals lobar consolidation with air bronchograms in bacterial pneumonia or interstitial infiltrates in viral or atypical pneumonia. Laboratory tests include CBC (leukocytosis), blood cultures, sputum culture, and sometimes urinary antigen tests for pneumococcus or Legionella. Severity scores like CURB-65 help determine need for hospitalization. CT chest may be needed if X-ray is negative but suspicion is high.
Is pneumonia curable?
Yes, pneumonia is curable with appropriate antibiotic therapy for bacterial pneumonia and supportive care for viral pneumonia. Most patients with community-acquired pneumonia improve within 3-5 days of starting antibiotics, with complete radiographic resolution in 4-6 weeks. Outpatient treatment is appropriate for healthy adults, typically with amoxicillin or a macrolide for 5-7 days. Hospitalized patients require intravenous antibiotics and supportive care. Overall mortality is less than 1% for outpatients and 5-15% for hospitalized patients, though this increases substantially with advanced age, severe illness, or hospital-acquired infection. Early diagnosis and appropriate antibiotics are key to good outcomes.
Pneumonia vs bronchitis — what is the difference?
The main difference is that pneumonia involves infection of the lung parenchyma (alveoli) with infiltrate on chest X-ray, while bronchitis is inflammation of the bronchi without parenchymal involvement and a normal chest X-ray. Pneumonia presents with fever, productive cough, dyspnea, and crackles on auscultation; chest X-ray shows consolidation. Bronchitis causes cough (productive or dry) without significant fever or dyspnea; lung sounds may be clear or show rhonchi. Pneumonia requires antibiotics if bacterial, whereas acute bronchitis is usually viral and treated supportively. The presence of infiltrate on imaging is the definitive distinguishing feature.
Key drugs
Related conditions and differentials
Sources and further reading
StatPearls (NCBI Bookshelf)MedlinePlusWHO ICD-10: J18.9
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.