1 Many types of bacteria may cause pneumonia. 2  Pneumonia caused by Mycoplasma pneumoniaeMycoplasma pneumoniae is sometimes mild and called  walking pneumonia 3  .. Viruses, such as influenza A (the flu virus) and respiratory syncytial virus (RSV) can cause pneumonia.iruses, bacteria, a fungus, or (in rare cases) parasites or other organisms can cause pneumonia. 1  In most cases, the specific organism (such as bacteria or virus) cannot be identified even with testing.