What causes pneumonia?
Pneumonia is an infection that inflames the air sacs—known as alveoli—in one or both lungs. It can be caused by a wide range of infectious agents, including bacteria, viruses, fungi, and, less commonly, certain parasites or non-infectious triggers such as aspiration of gastric contents or exposure to chemical irritants.
Bacterial pneumonia is most frequently caused by Streptococcus pneumoniae, though other pathogens like Haemophilus influenzae, Mycoplasma pneumoniae, Legionella pneumophila, and Staphylococcus aureus (particularly in post-viral or healthcare-associated cases) are also important causes. Viral pneumonia is commonly due to influenza A and B viruses, respiratory syncytial virus (RSV), SARS-CoV-2, adenovirus, and human metapneumovirus—especially in children and immunocompromised individuals. Fungal pneumonia tends to occur in people with weakened immune systems and may involve organisms such as Pneumocystis jirovecii, Histoplasma capsulatum, or Coccidioides immitis.
Risk factors that increase susceptibility include advanced age, young age (especially under 2 years), chronic lung disease (e.g., COPD, asthma), cardiovascular disease, diabetes mellitus, immunosuppression (from conditions like HIV/AIDS or medications such as corticosteroids or biologics), smoking, alcohol use disorder, and recent viral upper respiratory infections. Aspiration pneumonia arises when oropharyngeal secretions or gastric contents are inhaled into the lower airways—commonly in patients with dysphagia, altered mental status, or gastroesophageal reflux.
Diagnosis typically integrates clinical assessment (e.g., fever, cough, dyspnea, tachypnea, crackles on auscultation), imaging (chest X-ray or CT showing infiltrates or consolidation), and microbiologic testing when indicated (e.g., sputum culture, blood cultures, PCR assays, urinary antigen tests). Treatment depends on the likely pathogen, severity of illness, patient comorbidities, and local resistance patterns—and may include empiric or targeted antimicrobial therapy, supportive care, and hospitalization for moderate-to-severe cases.