Delirium (or the state of being delirious) is considered a temporary dysfunction of the brain.

The brain is a very complex organ, of course, and although it is resistant to being "knocked off kilter", certain things can do this.

• Many people have experienced delirium when drinking too much alcohol. This is induced by a chemical, is relatively temporary, and its effects are pretty well known. In too high a dose, the delirium can advance to unconsciousness, and even respiratory depression, which can lead to death from the body being unable to breathe on its own.
• Simple infections, like urinary infections in the elderly, can cause a delirium, which resolves a couple of hours after the first dose of antibiotic.

Delirium is confused with Dementia, but the two are quite different. See Dementia.