Gram-positive Streptococcus pneumoniae () is the most common<unk>  cause of pneumonia in all age groups except newborn infants. Streptococcus pneumoniae is a Gram-positive bacterium that often lives in the throat of people who do not have pneumonia. 3D medical animation still showing Streptococcus pneumoniae<unk> (causative<unk> of much<unk>  and meningitis). Other important Gram-positive causes of pneumonia are Staphylococcus aureus () and Bacillus anthracis. Gram-negative Gram-negative<unk> are seen less frequently: Haemophilus influenzae (), Klebsiella pneumoniae (), Escherichia coli (), Pseudomonas aeruginosa (), Bordetella pertussis, and Moraxella catarrhalis are the most common. These<unk> often live in the gut and enter the lungs when contents of the gut (such as vomit or faeces) are inhaled. Pneumonia caused by Yersinia pestis is usually called pneumonic plague. Atypical Atypical<unk> causing pneumonia are Coxiella burnetii, Chlamydophila pneumoniae (), Mycoplasma pneumoniae (), and Legionella pneumophila. The term "atypical" does not relate to how commonly these organisms cause pneumonia, how well it responds to common antibiotics or how typical the symptoms are; it refers instead to the fact that these organisms have atypical or absent cell wall structures and do not take up Gram stain in the same manner as gram-negative and gram-positive organisms.
Bacterial pneumonia
Viral pneumonia
Fungal pneumonia
Atypical pneumonia
Dust pneumonia
Community-acquired pneumonia
Aspiration pneumonia
Pneumococcal pneumonia