Alzheimer disease most commonly affects older adults. But it can also affect people in their 30s or 40s. When Alzheimer disease occurs in someone younger than age 65, it is known as early-onset (or younger-onset) Alzheimer disease.

Encephalitis is inflammation of the brain. The inflammation causes the brain to swell, which leads to changes in neurological function. It results in mental confusion and seizures.

An epidural abscess is an infection that forms in the space between your skull bones and your brain lining (intracranial epidural abscess).

Epilepsy is a brain condition that causes a person to have seizures. It is one of the most common disorders of the nervous system.

Essential tremor (ET) is a neurological disorder. It causes your hands, head, trunk, voice, or legs to shake rhythmically. It is often confused with Parkinson disease. It is most common in people older than 65.