Pneumonia is an acute or chronic infection involving the pulmonary parenchyma.
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 the most common form of embolic pulmonary infection with injected drug use and tricuspid valve endocarditis. The main preventive measures are smoking cessation, and vaccination against influenza and S. pneumoniae. Its classic presentation is with cough and fever, with variable sputum production, dyspnoea, and pleurisy. Most patients have constitutional symptoms and many also have gastrointestinal symptoms. Clinical examination may reveal features indicative of the severity of respiratory compromise and (in some cases) of consolidation.