Pneumonia — Symptoms, Causes & Treatment | MyMedicPlus

Pneumonia is an acute inflammatory infection of the lung parenchyma — specifically the alveoli and surrounding lung tissue — causing alveolar filling with inflammatory exudate (fluid, white blood cells, and proteins) that impairs gas exchange and oxygenation. It is caused by bacteria, viruses, fungi, or aspiration of oropharyngeal contents, with the pattern of causative organisms depending critically on whether infection was acquired in the community, a healthcare setting, or a hospital. Pneumonia affects approximately 450 million people annually and causes 4 million deaths — the leading infectious cause of death worldwide across all age groups and the leading cause of death in children under 5 years, accounting for 14% of all deaths in that age group. Community-acquired pneumonia (CAP — pneumonia developing outside hospital or within 48 hours of admission in a non-hospitalised patient) is the most common clinical category, occurring at a rate of 5-12 cases per 1,000 adults per year. Hospital-acquired pneumonia (HAP — developing more than 48 hours after hospital admission) and ventilator-associated pneumonia (VAP — developing more than 48-72 hours after endotracheal intubation) carry substantially higher mortality (20-30% for HAP; 20-50% for VAP) due to multidrug-resistant organisms and the underlying severity of illness in hospitalised patients. Severity is assessed using the validated CURB-65 score (Confusion, Urea, Respiratory rate, Blood pressure, age 65 or over) — scores of 3-5 predict 17-57% 30-day mortality and mandate ICU assessment.

Bacterial community-acquired pneumonia: Streptococcus pneumoniae is the most common CAP pathogen in all age groups (20-30% of cases), causing typical lobar consolidation with high fever, rigors, and rusty sputum; Haemophilus influenzae (particularly non-typeable strains — common in COPD); atypical organisms — Mycoplasma pneumoniae (causes up to 20-40% of CAP in younger adults; bilateral perihilar shadowing; responds to macrolides or doxycycline; cold agglutinins in 50%), Chlamydophila pneumoniae, Legionella pneumophila (from contaminated air-conditioning cooling towers and hot-water systems; associated with hyponatraemia, lymphopenia, elevated LDH, and urinary Legionella antigen positivity — requires high-dose fluoroquinolone or azithromycin). Viral pneumonia: influenza A and B viruses (particularly H1N1, H3N2 subtypes); SARS-CoV-2 (COVID-19 — bilateral peripheral ground-glass opacities on CT); RSV (the most common cause of pneumonia in young children and a significant cause in the elderly and immunocompromised); adenovirus; parainfluenza. Fungal: Pneumocystis jirovecii pneumonia (PCP — immunocompromised, typically HIV with CD4 below 200; bilateral perihilar 'ground glass' on CXR; treated with high-dose co-trimoxazole 15-20mg/kg/day trimethoprim component). Hospital-acquired pneumonia (HAP) and ventilator-associated pneumonia (VAP): Gram-negative organisms (Pseudomonas aeruginosa, Klebsiella pneumoniae, Acinetobacter baumannii — commonly multidrug-resistant) and MRSA dominate. Aspiration pneumonia: caused by aspiration of oropharyngeal flora (mixed anaerobes, streptococci) in patients with impaired consciousness, dysphagia, or neurological disease — typically affects right lower and middle lobes from anatomical gravity. Risk factors: extreme of age (over 65, under 5), current smoking (impairs mucociliary clearance), COPD, diabetes mellitus, liver cirrhosis, CKD, malignancy, immunosuppression (HIV, transplant, chemotherapy, corticosteroids), alcohol excess, prior influenza, recent hospitalisation, aspiration risk, and unvaccinated status for Streptococcus pneumoniae or influenza.

Classic 'typical' bacterial pneumonia (usually Streptococcus pneumoniae): acute onset over 24-48 hours of high-grade fever (above 38.5°C), rigors, productive cough with purulent yellow-green or 'rusty' blood-tinged sputum (from alveolar haemorrhage in pneumococcal lobar pneumonia), pleuritic chest pain (sharp, stabbing, worse with breathing and coughing — from pleural inflammation adjacent to the parenchymal consolidation), breathlessness, and malaise. 'Atypical' pneumonia (Mycoplasma, Chlamydophila, Legionella): more insidious onset over days to weeks; dry non-productive cough; headache; prominent myalgia; lower fever; extrapulmonary features (erythema multiforme, haemolytic anaemia from cold agglutinins in Mycoplasma; hyponatraemia, hepatitis, diarrhoea in Legionella). Examination findings: reduced air entry (fluid filling alveoli obstructs sound); bronchial breathing (harsh tubular breath sounds from lung consolidation transmitting tracheal sounds); dullness to percussion over consolidated or effused areas; coarse crackles (crepitations) from fluid in small airways; tachycardia; tachypnoea (respiratory rate above 20 is a key clinical severity marker). In elderly patients: atypical presentations are extremely common — new confusion (delirium), falls, incontinence, reduced appetite and functional decline without prominent respiratory symptoms or fever; this delays diagnosis and contributes to the high mortality of pneumonia in the elderly. Children under 5 years: rapid breathing, grunting, nasal flaring, subcostal and intercostal recession (chest-wall indrawing), and cyanosis may appear without prominent fever or productive cough. CURB-65 severity assessment: Confusion (AMTS below 8); Urea above 7 mmol/L; Respiratory rate 30/min or above; Blood pressure systolic below 90 or diastolic below 60 mmHg; age 65 or above — scores of 3-5 indicate severe CAP with 17-57% 30-day mortality requiring ICU assessment.

Chest X-ray (CXR): the standard diagnostic imaging for pneumonia — demonstrates lobar or segmental homogeneous consolidation (air-space opacity, air bronchograms) in typical bacterial pneumonia; bilateral perihilar interstitial infiltrates in atypical pneumonia (Mycoplasma) and PCP; and bilateral peripheral ground-glass opacities in COVID-19; a pleural effusion on CXR occurs in up to 40% of pneumonia (parapneumonic effusion — needs sampling if large or persistent to exclude empyema); normal CXR in early pneumonia does not exclude the diagnosis in the appropriate clinical context. CT thorax: more sensitive than CXR — identifies consolidation not visible on plain film, early cavitation (lung abscess from anaerobic pneumonia or Staphylococcus aureus), interstitial pattern versus consolidation, and pleural empyema; used in severe, atypical, or immunocompromised presentations. Blood investigations: full blood count (raised neutrophil leucocytosis suggests bacterial cause; lymphopenia common in viral pneumonia including COVID-19 and Legionella); CRP (markedly elevated above 100mg/L suggests bacterial CAP; procalcitonin [PCT] above 0.25 mcg/L differentiates bacterial from viral pneumonia with reasonable specificity and guides antibiotic stewardship); renal profile (urea for CURB-65 calculation; AKI in severe CAP); LFTs (elevated transaminases in Legionella and Mycoplasma); ABG/SpO2 (oxygen saturation and hypoxaemia assessment for HFNO or NIV need); LDH (elevated in PCP). Blood cultures: 2 sets before antibiotic administration in all hospitalised CAP patients — positive in 10-15% of inpatient CAP, most commonly S. pneumoniae; essential in HAP/VAP. Sputum culture and sensitivity: expectorated sputum sample or BAL (bronchoalveolar lavage) in ventilated patients — guides antibiotic de-escalation and MRSA detection. Urinary antigens: Legionella pneumophila serogroup 1 urinary antigen (sensitivity 70-90%, specificity above 95% — remains positive for weeks after infection onset; test in all moderate-severe CAP or with hyponatraemia/diarrhoea); pneumococcal urinary antigen (sensitivity 70-80% in bacteraemic pneumococcal CAP). Respiratory virology: nasopharyngeal swab or BAL for multiplex PCR panel (influenza A/B, SARS-CoV-2, RSV, adenovirus, parainfluenza, human metapneumovirus); Mycoplasma IgM serology (positive from day 7 of illness); SARS-CoV-2 PCR in the current clinical context.

Treatment of CAP is guided by severity stratification using CURB-65. Low-risk CAP (score 0-1): outpatient management — amoxicillin 500mg three times daily for 5 days (NICE and BTS guidelines) for typical bacterial pneumonia; doxycycline 200mg loading dose then 100mg twice daily (5 days) for penicillin-allergic patients or suspected atypical infection (Mycoplasma, Chlamydophila); amoxicillin plus doxycycline or clarithromycin in combination for dual coverage when atypical organism is considered alongside typical bacterial CAP. Moderate CAP (score 2): hospital admission recommended — IV amoxicillin 500mg-1g three times daily plus IV or oral clarithromycin 500mg twice daily; switch to oral amoxicillin and clarithromycin when improving (temperature below 37.5°C, respiratory rate below 24, oxygen no longer required). Severe CAP (score 3-5): urgent ICU assessment — IV co-amoxiclav (piperacillin-tazobactam for high-risk HAP organisms) plus IV clarithromycin; alternative regimen: IV ceftriaxone plus IV azithromycin; or IV levofloxacin monotherapy for penicillin-allergic patients. Specific pathogen-directed therapy: Legionella pneumophila — fluoroquinolone (levofloxacin 500mg BD) or macrolide; Pneumocystis jirovecii pneumonia (PCP) in HIV or immunocompromised patients — high-dose co-trimoxazole (trimethoprim 15-20mg/kg/day in divided doses) plus adjunctive corticosteroids (prednisolone 40mg twice daily for 5 days for moderate-severe PCP with PaO2 below 9.3kPa). Hospital-acquired/ventilator-associated pneumonia: broader-spectrum empirical therapy (piperacillin-tazobactam, carbapenems for resistant organisms) guided by local microbiology and antibiogram. Supportive care: supplemental oxygen to maintain SpO2 94-98% (88-92% in COPD to avoid hypercapnic drive suppression); IV fluids for sepsis-related hypotension; high-flow nasal oxygen (HFNO) or NIV (CPAP for ARDS) for acute hypoxaemic respiratory failure avoiding intubation; prone positioning for severe ARDS.

Respiratory failure from extensive bilateral consolidation reduces gas exchange, causing hypoxaemia requiring supplemental oxygen; progression to acute respiratory distress syndrome (ARDS) — characterised by bilateral infiltrates, PaO2/FiO2 ratio below 300, and non-cardiogenic pulmonary oedema — occurs in 5-10% of hospitalised CAP patients, carries 30-40% mortality, and requires invasive mechanical ventilation with lung-protective strategy (6mL/kg tidal volume, PEEP titration). Parapneumonic effusion develops in up to 40% of hospitalised pneumonia patients — a reactive exudative pleural effusion from inflammation; complicated parapneumonic effusion (pH below 7.2, glucose below 2.2 mmol/L, LDH above 1000) and empyema thoracis (pus in the pleural space) require pleural drainage (chest drain or VATS) plus prolonged antibiotics — failure to drain leads to fibrothorax and chronic respiratory impairment. Sepsis and septic shock: pneumonia is the most common infection precipitating sepsis — systemic inflammatory response causing multi-organ failure; septic shock (MAP below 65mmHg despite adequate fluid resuscitation) requires vasopressors (noradrenaline) and intensive care support. Lung abscess: cavitating necrosis within consolidated lung from virulent organisms (Staphylococcus aureus, Klebsiella pneumoniae, anaerobes in aspiration) — requires prolonged antibiotic therapy (4-6 weeks) and occasional surgical drainage. Post-pneumonia sequelae: persistent radiographic changes (up to 6 weeks); impaired lung function on spirometry in 25-30% of survivors at 6 months; markedly elevated risk of subsequent cardiovascular events — MI risk is 17-fold higher in the 30 days following hospitalisation for pneumonia; and long-term cognitive impairment in elderly survivors of severe pneumonia.

Seek emergency care (999/112 or A&E) immediately for: confusion or altered consciousness (new onset), respiratory rate above 30 breaths per minute, systolic blood pressure below 90 mmHg, SpO2 below 92% on pulse oximetry, or cyanosis (blue lips or fingertips) — these features indicate severe pneumonia and possible sepsis requiring immediate hospital assessment. Also seek emergency care for: chills with rigors, pleuritic chest pain with breathlessness, inability to swallow oral medications or fluids. Use the CRB-65 score at home: Confusion (new), Respiratory rate above 30/min, Blood pressure below 90/60 mmHg, age 65 or over — score 2 or above requires urgent hospital assessment. See a GP same-day for: high fever with productive cough, significant breathlessness worse than usual, or failure to improve after 48-72 hours of antibiotics for community-acquired pneumonia. All patients treated at home should have a follow-up chest X-ray at 6-8 weeks to confirm radiological resolution — non-resolving pneumonia (particularly in smokers over 40) requires investigation to exclude an underlying lung malignancy causing post-obstructive pneumonia.

Pneumococcal vaccination: PCV13 (Prevenar 13) or PCV15 (Vaxneuvance) for all children at 8 weeks, 16 weeks, and 1 year in the UK schedule; PPSV23 (Pneumovax 23) for all adults aged 65 and above and for immunocompromised patients (asplenia, CKD, diabetes, HIV, transplant recipients) at any age — together these vaccines reduce invasive pneumococcal disease and hospitalisation for CAP by 40-60% in vaccinated populations. Annual influenza vaccination for all adults over 65 and high-risk groups (chronic respiratory disease, cardiovascular disease, DM, immunosuppression, pregnancy) significantly reduces secondary bacterial pneumonia occurring post-influenza. COVID-19 vaccination reduces severe pneumonia from SARS-CoV-2 infection. RSV vaccination (RSVPreF3, mRNA-1345 — approved 2023-24 for adults 60+) reduces RSV-associated lower respiratory tract infection, particularly important in the elderly and immunocompromised. Smoking cessation: smoking impairs mucociliary clearance, phagocytic function, and alveolar macrophage activity — cessation markedly reduces infection susceptibility and pneumonia hospitalisation risk. Alcohol reduction: excessive alcohol impairs immune function and aspiration risk. Hospital infection prevention: strict hand hygiene (alcohol gel before and after patient contact); isolation precautions for droplet-transmissible pathogens (influenza, COVID-19, Mycoplasma); ventilator care bundles for VAP prevention (head-of-bed elevation to 30-45 degrees, daily sedation breaks, oral decontamination with chlorhexidine 0.12% gel, subglottic suction endotracheal tubes, tracheal cuff pressure monitoring above 20cmH2O) — these bundles reduce VAP incidence by 40-70%. Aspiration prevention: upright positioning during feeding; thickened fluids for dysphagia identified by SALT assessment; nasogastric or PEG tube feeding in patients with severe aspiration risk.