Dementia refers to a problem with thinking skills like remembering, paying attention, multi-tasking that is bad enough that it makes it hard to do everyday activities like having a conversation, preparing a meal, planning your day, or doing chores.

Dementia can be fixed (reversed) if it is caused by an infection (like syphilis) or a poisoning (like overdose of bismuth from taking too much pepto-bismal) but in most cases it is not.

The top three most common causes are

1. Alzheimer's disease
2. Vascular
3. Lewy body (which includes dementia with Lewy bodies or Parkinson's disease dementia)

The diagnosis of dementia is made by a clinician (such as doctor or nurse practitioner) by getting the whole story and completing the necessary tests (this may include testing by a neuropsychologist.) Doctors and other clinicians who diagnose and treat dementia regularly (for example neurologists, geriatric medicine specialists, or geriatric psychiatrists) will use a combination of the history, neurological exam, brain scans, blood tests, and the neuropsychological test results to diagnose the cause of dementia based on standard criteria.