Accumulation of pus in a body cavity, as opposed to an abscess, which develops within tissue. It can occur in the nasal sinuses, gall bladder, pulmonary cavity, abdominal cavity, marrow cavity of bones, lachrymal glands and so on. Pus consists of dead tissue, dead and living bacteria, and white blood cells attempting to destroy the bacteria. There is thus always infection, caused by the closure of the cavity and spreading of infection within it. The commonest form is sinus empyema or sinusitis , often following a cold in the nose. Infection spreads to the sinuses, which close as a result of swelling of the mucous membrane. Symptoms depend on the place affected, but almost always include redness, pain, disturbed function and swelling if the infection is deep in the cavity. Sometimes general malaise and fever occur. The danger is that the condition can spread, and cause perforation of weak spots, notoriously in the gall bladder. The diagnosis is by general physical examination, blood tests, ultrasonic scans and X-rays, and treatment is on the principle that where there is pus it must be removed – by surgery from the gall bladder, for example, or by lancing a sinus. Thus a specialist may be required. Measures can also be taken to increase resistance, and antibiotics given to prevent the infection spreading.