What causes pneumonia?

    Written by:
    Louie Neri

    Fact-checked

    by:

    Rachel Hathcock, James Gaylor
    Last Updated: May 16, 2025

    Pneumonia can be caused by various pathogens, including bacteria (like Streptococcus pneumoniae), viruses (such as influenza and RSV), and fungi (like Pneumocystis jirovecii), and the prevalence of these microbes varies by geography and age group. Additionally, aspiration pneumonia can occur when food, liquids, or gastric contents enter the lungs.

    Many types of pathogens can cause pneumonia, such as:

    • Bacteria: Streptococcus pneumoniae, Haemophilus influenzae, Mycoplasma pneumoniae, Legionella pneumophila, and methicillin-resistant Staphylococcus aureus (MRSA)

    • Viruses: Human rhinovirus, influenza A or B, coronavirus, and respiratory syncytial virus (RSV)

    • Fungi: Pneumocystis jirovecii, Aspergillus spp., and Mucorales

    The most prevalent causative microbes vary geographically. Viruses account for the majority of cases in children (<5 years)[1], whereas bacteria and viruses account for most cases of pneumonia in adults. Fungal pneumonia is quite rare and usually occurs in people who are immunocompromised (e.g., cancer patients).[2]

    Pneumonia can also be caused by foods, liquids, or gastric contents inadvertently entering the lungs, a condition known as aspiration pneumonia.

    What causes pneumonia? - Examine