The most commonly identified organism is Streptococcus pneumoniae. Other organisms that have been identified include Haemophilus influenza, atypical bacteria and viruses.
Viruses, bacteria, and fungi can all cause pneumonia. In the United States, common causes of viral pneumonia are influenza, respiratory syncytial virus (RSV), and SARS-CoV-2 (the virus that causes COVID-19). A common cause of bacterial pneumonia is Streptococcus pneumoniae (pneumococcus).