Pneumonia can be caused by several microorganisms , but the most common form is bacterial pneumonia  caused by the bacteria Streptococcus pneumoniae. Other bacteria that can cause pneumonia include Haemophilis influenza and Staphylococcus aureus.

Other forms of pneumonia include:

• Viral: Respiratory Syncytial Virus (RSV) is the most common cause of viral pneumonia
• Fungal: It is rare but more likely to affect people with a weakened immune response
• Aspiration: Caused by substances such as vomit, food, dust etc entering the lungs and causing an infection.  Most common in people with a compromised gag reflex.