Aetiology—most cases are caused by microbial pathogens, the commonest being Streptococcus pneumoniae, Haemophilus influenzae, Mycoplasma pneumoniae, Chlamydia pneumoniae, legionella, anaerobic bacteria, and viruses (influenza, parainfluenza, and respiratory syncytial virus). Staphylococcus aureus is an important superinfecting pathogen in influenza, and is the most common form of embolic pulmonary infection with injection-drug use and tricuspid valve endocarditis.