Gram-positive
Streptococcus pneumoniae
Staphylococcus aureus
Gram-negative
Haemophilus influenzae
Klebsiella pneumoniae
Escherichia coli
Pseudomonas aeruginosa 
Moraxella catarrhalis
Atypical pneumonia
Chlamydophila pneumoniae 
Chlamydophila psittaci
Mycoplasma pneumoniae
Coxiella burnetti 
Legionella pneumophila
Streptococcus pneumoniae
Staphylococcus aureus
Haemophilus influenzae
Klebsiella pneumoniae
Escherichia coli
Pseudomonas aeruginosa 
Moraxella catarrhalis
Chlamydophila pneumoniae 
Chlamydophila psittaci
Mycoplasma pneumoniae
Coxiella burnetti 
Legionella pneumophila
Common
Influenza<unk>  syncytial virus
Parainfluenza
Rarer
Adenovirus
Metapneumovirus
Severe acute<unk><unk>  (SARS)<unk>  (MERS)
2019-nCoV (COVID-19)
Cause other diseases, but sometimes cause pneumonia
Herpes simplex virus
Varicella-zoster (VZV)
Measles
Rubella
Cytomegalovirus
Smallpox
Dengue
Influenza<unk>  syncytial virus
Parainfluenza
Adenovirus
Metapneumovirus
Severe acute<unk><unk>  (SARS)<unk>  (MERS)
2019-nCoV (COVID-19)
Herpes simplex virus
Varicella-zoster (VZV)
Measles
Rubella
Cytomegalovirus
Smallpox
Dengue
Histoplasmosis
Coccidioidomycosis
Blastomycosis
Pneumocystis jirovecii pneumonia (PCP)
Sporotrichosis
Cryptococcosis
Aspergillosis
Candidiasis
Ascariasis
Schistosomiasis
Toxoplasma gondii
Middle East respiratory syndrome coronavirus
Gram-positive
West Nile virus
Streptococcus pneumoniae
Enterovirus D68
Staphylococcus aureus
Heartland virus
Gram-negative