Anaphylaxis.

Anaphylaxis is the most severe form of allergic reaction. Unlike a mild reaction (a runny nose, an itchy rash, watery eyes), anaphylaxis involves the airway, breathing or circulation, and can kill within minutes if it is not treated.

The hallmark signs are any one of: swelling of the tongue or throat, difficulty breathing or noisy breathing, persistent cough or wheeze, a hoarse voice, collapse, or pale floppiness in a young child. Hives and flushing often appear too, but they are not required for the diagnosis — the airway / breathing / circulation involvement is.

First-line treatment is adrenaline by intramuscular auto-injector into the outer mid-thigh, with the casualty laid flat (or sat up only if breathing is hard) and an ambulance called immediately. A second dose may be given after five minutes if the casualty has not improved.