Micro organisms in Pnuemonia

Viruses
Viral pneumonia is commonly caused by viruses such as influenza virus, respiratory syncytial virus (RSV), adenovirus, and metapneumovirus. Herpes simplex virus is a rare cause of pneumonia except in newborns. People with weakened immune systems are also at risk of pneumonia caused by cytomegalovirus (CMV).
Bacteria
The most common causes of bacterial pneumonia are Streptococcus pneumoniae, Gram-positive bacteria and "atypical" bacteria. The term "atypical" is used because atypical bacteria commonly affect healthier people, cause generally less severe pneumonia, and respond to different antibiotics than other bacteria.
Streptococcus pneumoniae, often called "pneumococcus", is the most common bacterial cause of pneumonia in all age groups except newborn infants.
"Atypical" bacteria which cause pneumonia include Chlamydophila pneumoniae, Mycoplasma pneumoniae, and Legionella pneumophila. Another important Gram-positive cause of pneumonia is Staphylococcus aureus, with Streptococcus agalactiae being an important cause of pneumonia in newborn babies. Gram-negative bacteria cause pneumonia less frequently than gram-positive bacteria. Some of the gram-negative bacteria that cause pneumonia include Haemophilus influenzae, Klebsiella pneumoniae, Escherichia coli, Pseudomonas aeruginosa and Moraxella catarrhalis. These bacteria often live in the stomach or intestines and may enter the lungs if vomit is inhaled.