David Cassidy Has Dementia: Here's What That Means
David Cassidy at a concert in 2015.
Credit: Debby Wong/Shutterstock

Actor and singer David Cassidy recently revealed he has dementia, but what exactly does this term mean?

Cassidy, who is 66, told People magazine on Monday (Feb. 20) that he has dementia, and will stop touring as a musician because of his diagnosis. The actor also said that both his mother and grandfather suffered from dementia.

"I was in denial, but a part of me always knew this was coming," Cassidy said.

Dementia is not a specific disease, but rather a group of symptoms resulting from changes in the brain that affect people's ability to carry out everyday activities, according to Mayo Clinic. These symptoms can include memory loss, problems with communication (such as trouble engaging in conversation), difficulty solving problems and reasoning, or changes in personality. Dementia is caused by damage to the brain's nerve cells, and the types of symptoms people have can depend on the part of the brain that's affected, according to Mayo Clinic. [6 Big Mysteries of Alzheimer's Disease]

Doctors diagnose dementia when people have impairments in at least 2 out of 5 core brain functions, according to the Alzheimer's Association. These five core functions are memory, communication and language, ability to focus, reasoning and judgment, and visual perception (such as the ability to judge distance or determine the height of a stair).

People's risk for dementia increases as they get older, and those over age 65 are at particularly high risk, according to Mayo Clinic. Those with a family history of dementia are also at greater risk for the condition.

There are many diseases and conditions that can cause dementia, including the following:

Alzheimer's disease. This is the most common type of dementia, accounting for up to 80 percent of dementia cases, according to the Alzheimer's Association. A hallmark of Alzheimer's disease in the brain is the buildup of proteins known as beta-amyloid and tau.

Vascular dementia. This is the second most common type of dementia, accounting for about 10 percent of cases, according to the Alzheimer's Association. It is caused by damage to the brain's blood vessels, which can occur after a stroke.

Lewy body dementia. Abnormal clumps of a protein called alpha-synuclein cause this type of dementia, according to the Alzheimer's Association. In addition to memory loss, people with Lewy body dementia may also experience sleep disturbances, visual hallucinations and problems with movement. After his death in 2014, actor Robin Williams was found to have Lewy body dementia.

Parkinson's disease. People with Parkinson's disease often experience symptoms of dementia as the disease progresses. It is caused by the breakdown of nerve cells that produce a brain chemical called dopamine.

Frontotemporal dementia. This is a group of dementias that involves the breakdown of nerve cells in the brain's frontal and temporal lobes, according to Mayo Clinic. It can cause changes in personality and behavior. People with this type of dementia generally start showing symptoms at younger ages (around age 60), according to the Alzheimer's Association.

Mixed dementia. This occurs when people have the brain abnormalities of several different types of dementia at the same time. For example, people may have brain abnormalities linked with Alzheimer's disease, vascular dementia and Lewy body dementia.

Most dementias are progressive disorders, which means the symptoms get worse over time. But some symptoms of dementia are reversible. For example, thyroid problems, exposure to heavy metals and reactions to certain medications can cause symptoms of dementia that are reversed with treatment, according to Mayo Clinic.

Cassidy did not specify which type of dementia he has. But he has spoken at events for the Alzheimer's Foundation of America in order to raise awareness about dementia, according to AgingCare.com. His mother, Evelyn Ward, died from an Alzheimer's-related dementia, according to the Hollywood Reporter.

Original article on Live Science.