Bacterial causes of pneumonia vary by population, but commonly discussed organisms in clinical practice include Streptococcus pneumoniae, Haemophilus influenzae, Staphylococcus aureus, Klebsiella pneumoniae, and—within the atypical group—Mycoplasma pneumoniae. This is not a universal ranking; distributions change with age, comorbidities, smoking status, post-influenza periods, and local resistance patterns. For example, Staphylococcus aureus risk can rise after influenza, and Klebsiella may cause more severe disease in certain risk groups. The operational goal is to align likely pathogens with patient risk and local data to guide empiric decisions and refine as new information emerges.