Pneumonia is a serious lung infection that affects millions of people worldwide, ranging from mild cases to life-threatening conditions. It occurs when the air sacs in one or both lungs become inflamed and fill with fluid or pus, making it difficult for oxygen to reach the bloodstream. This inflammation can result from various pathogens, including bacteria, viruses, and fungi. Pneumonia can occur as a complication of another illness or as a primary infection, and it affects people of all ages, though certain groups, such as the elderly, young children, and individuals with compromised immune systems, are at higher risk.

Understanding pneumonia starts with recognizing the different types of the disease. The most common types are bacterial, viral, and fungal pneumonia, each caused by different pathogens and each requiring distinct treatment approaches. Bacterial pneumonia is typically caused by Streptococcus pneumoniae, which is the most common bacterial pathogen responsible for hospitalizations. Other bacteria, such as Haemophilus influenzae and Staphylococcus aureus, can also lead to pneumonia. Viral pneumonia, often caused by influenza viruses, respiratory syncytial virus (RSV), and coronaviruses, is another common form, and its severity can vary based on the viral strain and the individual's overall health. Fungal pneumonia is less common and typically occurs in individuals with weakened immune systems, with fungi like Histoplasma, Coccidioides, and Cryptococcus causing infections.