Infection with the Streptococcus pneumoniae bacterium, also called pneumococcus, is the most common cause of bacterial pneumonia. Pneumococcus usually causes lobar pneumonia, attacking an entire lobe or portion of a lobe of the lung; in double pneumonia, pneumococcus attacks both lungs. Pneumococcal lobar pneumonia often occurs in winter after an acute, viral upper respiratory infection. Usual symptoms include a shaking chill followed by a fever of about 40°C (104°F), pain in the chest while breathing, a cough, and blood-streaked sputum.

 Other bacteria that cause pneumonia include Klebsiella pneumoniae, Haemophilus influenzae, Legionella pneumophilia (the bacterium that causes Legionnaires’ disease), and various staphylococci and streptococci bacteria. Infections with these organisms primarily cause bronchopneumonia. Onset of symptoms is generally slower than with lobar pneumonia, and the fever is lower.