What is dementia?

Dementia is not one disease but a term for a decline in memory, thinking or reasoning severe enough to interfere with daily life. It is caused by physical changes in the brain, and several diseases can cause it. Alzheimer's disease is the most common; vascular, Lewy body and frontotemporal dementia account for most of the rest, and many people have more than one.

Dementia is not a single disease. The Alzheimer's Association calls it a general term for loss of memory, language, problem solving and other thinking abilities severe enough to interfere with daily life, caused by a range of diseases. Alzheimer's disease is the most common. Knowing the specific type matters because symptoms, safe medications and progression differ, and a doctor can help identify it.

In the brain, dementia is caused by diseases that damage brain cells. The Alzheimer's Association explains that this damage interferes with the ability of brain cells to communicate with each other, which affects thinking, behavior and feelings. In Alzheimer's disease, for example, high levels of certain proteins inside and outside brain cells make it hard for those cells to stay healthy.

No. Dementia is the general term for a decline in memory and thinking that interferes with daily life, while Alzheimer's disease is one specific brain disease that causes it. Alzheimer's accounts for 60 to 80 percent of cases, according to the Alzheimer's Association, which is why the two words are often used interchangeably. Other causes include vascular, Lewy body and frontotemporal dementia.

Most dementia develops gradually over months or years. Vascular dementia is a partial exception: MedlinePlus notes its symptoms may begin suddenly after a stroke. Confusion that appears over hours or days is more likely to be delirium, which is often caused by an infection or medication and can usually be treated. Anyone with a sudden change in thinking should see a doctor right away.