'Never been the same since'
Delirium in older people might have permanent effects on the brain.
If you look up delirium in a medical dictionary, the definition refers to an altered state of consciousness that includes confusion, distractibility, disordered memory, and hallucinations. Causes include diseases that directly affect the brain, like meningitis, and withdrawal from alcohol.
But starting at about age 65, delirium has any number of causes — an infection, a fall, a fracture, dehydration, a surgical procedure — that aren't so obviously related to the brain. Physically, people can be agitated and restless, or just the opposite, lethargic and listless. Family and friends are often bewildered because people swing from being lucid and seemingly back to their old selves to being utterly confused and "out of it."