Aspiration: when the wrong things go down
Normally a precise reflex seals the larynx when you swallow, so food and drink go to the stomach, not the lungs. When that protection fails — during reduced consciousness, a stroke, heavy alcohol use, or trouble swallowing — saliva or stomach contents can enter the airways. This is aspiration, and when infection follows it becomes aspiration pneumonia. Because we usually aspirate while lying down, it tends to settle in the lower or back parts of the lung that are lowest at that moment.
Aspirated material carries a mix of mouth bacteria, including anaerobes that thrive without oxygen. Left untreated, a pocket of this infection can liquefy a patch of lung into a cavity of pus — a lung abscess. An abscess produces large amounts of foul-smelling sputum and shows as a round cavity, sometimes with a fluid level, on imaging.
Empyema: pus outside the lung
Pneumonia near the surface of the lung often draws fluid into the pleural space — the thin gap between lung and chest wall. A simple reactive pleural effusion may clear on its own. But if bacteria seed that fluid, it turns into thick pus: an empyema. Unlike pneumonia, an empyema cannot be cured by antibiotics alone, because drugs penetrate a walled-off pocket of pus poorly. It must be physically drained.
Doctors sample the fluid with a needle (thoracentesis) and analyse it. An infected effusion is an exudate — protein-rich and inflammatory — with low pH and glucose and possibly visible pus. The treatment is a chest tube to drain the space, plus antibiotics, and sometimes surgery to peel away the rind that forms.
Hospital-acquired pneumonia and resistant bugs
Hospital-acquired pneumonia develops 48 hours or more after admission. It matters because hospital bacteria are different from community ones — more likely to be resistant strains such as MRSA or Pseudomonas. The most feared form is ventilator-associated pneumonia, which arises in patients on a breathing machine: the tube bypasses the throat's natural defences and lets bacteria track straight down into the lungs.
Because the likely organisms are tougher, hospital pneumonia is treated with broader, stronger antibiotics chosen with local resistance patterns in mind — then narrowed once cultures identify the exact bug. Prevention is just as important: keeping the head of the bed raised, careful mouth care, and getting patients off ventilators as soon as it is safe.