Pneumonia is primarily caused by infections from bacteria, viruses, or fungi that enter the lungs. The most common way to get pneumonia is when these germs are inhaled into the lungs, often following or during another respiratory infection like a cold or the flu. Bacterial pneumonia, often caused by Streptococcus pneumoniae, is common, but viruses such as the flu virus, respiratory syncytial virus (RSV), and even SARS-CoV-2 (COVID-19) can also cause it. Fungal pneumonia occurs mostly in individuals with weakened immune systems. Pneumonia can also occur due to aspiration, where food, drink, or harmful substances accidentally enter the lungs. Additionally, people receiving hospital care, particularly those on ventilators, are at risk of pneumonia. Risk factors include being very young or elderly, having chronic health problems, smoking, or having a weakened immune system. Pneumonia germs can be spread from person to person via respiratory droplets. Good hygiene practices and vaccines (e.g., pneumococcal and flu vaccines) can help reduce the risk of pneumonia. In summary, pneumonia occurs when harmful germs infect the lungs, often after inhaling them from the air or following another illness that weakens lung defenses.
