Delirium is a sudden, temporary onset of an altered mental state. The length of the episodes can vary from days to weeks. It is more common in older adults, and often follows an illness, surgery, or a response to a medication. Hospital stays can often result in delirium, and people with underlying medical issues are at higher risk. Dementia, on the other hand, has a more gradual onset and is not temporary. It is possible for this to happen to your sister again. Because she has had this experience, you can be aware of what to watch for and keep her doctor informed when you see symptoms.