Pneumonia develops when microorganisms—most commonly bacteria or viruses, and less frequently fungi—infect the lungs and trigger inflammation. In community-acquired pneumonia, frequent causes include Streptococcus pneumoniae, respiratory viruses (such as influenza), and “atypical” organisms like Mycoplasma pneumoniae. Risk rises with older age, chronic lung or heart disease, diabetes, immunosuppression, smoking, poor nutrition, and recent viral illness. Aspiration (inhaling stomach contents or oropharyngeal secretions) is another major pathway. Identifying the most likely cause and assessing risk severity are key to choosing the right treatment strategy.