Which bacterium is the most common cause of bacterial pneumonia?

The bacterium most commonly responsible for bacterial pneumonia is Streptococcus pneumoniae, also known as the pneumococcus. It is a Gram-positive, encapsulated bacterium that can colonize the upper respiratory tract without causing illness, yet sometimes reaches the lungs and produces pneumonia.

Pneumococcal pneumonia often follows a viral respiratory infection, which can damage airway defenses and make bacterial invasion easier. The infection may cause fever, cough, chest pain, shortness of breath, and sputum production. Streptococcus pneumoniae can also cause meningitis, bloodstream infections, and middle-ear infections, making it an important pathogen beyond the lungs.

The wording “most common” describes the leading bacterial cause overall, not the cause of every pneumonia case. Viruses are also major causes of pneumonia, and other bacteria—including Haemophilus influenzae, Staphylococcus aureus, and atypical organisms such as Mycoplasma pneumoniae—can be responsible. Vaccines targeting pneumococcal capsules have substantially reduced severe disease, especially in young children and older adults.