Delirium (acute confusional state) is a sudden change in a person's mental status, leading to confusion and unusual behavior.

Symptoms of delirium usually develop over the course of several hours to a few days. Symptoms may fluctuate and often include:

• Confusion and disorientation, including hallucinations, delusions, or illusions.
• Disorganized thinking.
• Short attention span.
• Problems with the sleep-wake cycle.
• Periods of hyperactivity alternating with extreme sleepiness.

Delirium may be caused by withdrawal from alcohol or drugs or medicines. The development or worsening of an infection or other health problem can also lead to delirium. People with dementia, such as occurs with Alzheimer's disease, are more prone to delirium.