Pneumonia is an infection of one or both lungs caused by bacteria, viruses, fungi, or chemical irritants. The most common causes of pneumonia are bacteria and viruses in the air we breathe. Some of the most common bacteria that cause pneumonia include Streptococcus pneumoniae, which is responsible for pneumococcal pneumonia, and Mycoplasma pneumoniae, which causes atypical pneumonia. Other bacteria that can cause pneumonia include Chlamydophila pneumoniae and Legionella pneumophila. Viruses that can cause pneumonia include the flu virus, respiratory syncytial virus (RSV), and SARS-CoV-2, the virus that causes COVID-19. Fungi can also cause pneumonia, with the most common type being caused by the fungus Pneumocystis jirovecii.