What are the causes of pneumonia?

Medical Advisory BoardAll articles are reviewed for accuracy by our Medical Advisory Board
Educational purpose only • Exercise caution as content is pending human review
Article Review Status
Submitted
Under Review
Approved

Last updated: April 1, 2025View editorial policy

Personalize

Help us tailor your experience

Which best describes you? Your choice helps us use language that's most understandable for you.

From the Guidelines

Pneumonia is typically acquired through the inhalation of airborne droplets containing infectious agents, such as bacteria, viruses, or fungi, or by aspirating food, liquid, or vomit containing bacteria into the lungs. The most common bacterial cause of pneumonia is Streptococcus pneumoniae, while viral causes include influenza and respiratory syncytial virus (RSV) 1. According to a study published in 2021, S. pneumoniae remains the most common bacterial pathogen responsible for community-acquired pneumonia, regardless of age and comorbidities 1.

Common Causes of Pneumonia

  • Bacterial causes: Streptococcus pneumoniae, Haemophilus influenzae, and Staphylococcus aureus
  • Viral causes: Influenza, respiratory syncytial virus (RSV), and coronavirus
  • Fungal causes: Pneumocystis jirovecii and Aspergillus fumigatus

Risk Factors for Developing Pneumonia

  • Age (very young or elderly)
  • Smoking
  • Chronic diseases like COPD or diabetes
  • Weakened immune system
  • Hospitalization

Prevention and Treatment

  • Get vaccinated with pneumococcal vaccines (Prevnar 13 and Pneumovax 23) and annual flu shots
  • Practice good hand hygiene
  • Avoid smoking
  • Maintain overall health
  • Seek medical attention promptly if symptoms like fever, cough with phlegm, chest pain, and difficulty breathing occur, as pneumonia can become serious, especially in vulnerable populations 1.

From the Research

Causes of Pneumonia

  • Pneumonia is a common acute respiratory infection that affects the alveoli and distal airways, and it is caused by a large variety of microorganisms, including bacteria, respiratory viruses, and fungi 2.
  • The development of the disease largely depends on the host immune response, with pathogen characteristics having a less prominent role 2.
  • Pneumonia occurs more commonly in susceptible individuals, including children of <5 years of age and older adults with prior chronic conditions 2.

Types of Pneumonia

  • Pneumonia is broadly divided into community-acquired pneumonia or hospital-acquired pneumonia 2.
  • Community-acquired pneumonia can be treated with antibiotics such as azithromycin or amoxicillin-clavulanate 3, 4.
  • Hospital-acquired pneumonia may be caused by more resistant bacteria and requires more aggressive treatment 5.

Risk Factors

  • Individuals with prior chronic conditions, such as respiratory failure, are at higher risk of developing pneumonia 5.
  • The use of intravenous azithromycin has been shown to have favorable effects in patients with influenza virus pneumonia using mechanical ventilation or oxygen 5.
  • Antibiotic therapy, such as amoxicillin/clavulanate or azithromycin, can eradicate or suppress strains of Streptococcus pneumoniae and Haemophilus influenzae, but may also allow for the spread of resistant strains 6.

Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

Have a follow-up question?

Our Medical A.I. is used by practicing medical doctors at top research institutions around the world. Ask any follow up question and get world-class guideline-backed answers instantly.