Glen Campbell's Death: How Does Alzheimer's Kill?
Glen Campbell arrives at the 2012 CMT Music awards at the Bridgestone Arena on June 6, 2012 in Nashville, Tennessee.
Credit: Rick Diamond/Getty Images for CMT

Country music legend Glen Campbell has died at age 81 following a long battle with Alzheimer's disease, his family announced today.

Campbell was diagnosed with Alzheimer's disease in 2011, at the age of 75, according to Rolling Stone. Later that year, he announced he was retiring because of his illness, and began a farewell tour that included 151 shows, Rolling Stone said.

Although Alzheimer's disease shortens people's life spans, it is usually not the direct cause of a person's death, according to the Alzheimer's Society, a charity in the United Kingdom for people with dementia. Rather, people die from complications from the illness, such as infections or blood clots.

Alzheimer's is a progressive brain disease in which abnormal protein deposits build up in the brain, causing brain cells to die. The illness is best known for causing memory loss, but it also has other debilitating effects on the body, and can affect people's ability to move and eat by themselves. There is no cure for the illness. [6 Big Mysteries of Alzheimer's Disease]

Alzheimer's patients may have difficulty swallowing, and they may inhale food, which can result in aspiration pneumonia, Dr. Marc L. Gordon, chief of neurology at Zucker Hillside Hospital in Queens, New York, who was not involved in Campbell's care, told Live Science in a 2014 interview. Pneumonia is listed as the cause of death in as many as two-thirds of deaths of patients with dementia, according to the Alzheimer's Society. 

Alzheimer's patients may also become bedridden, which can increase their risk of fatal blood clots, Gordon said.

Weight loss and other complications from Alzheimer's can also lead to a weakened immune system, according to the Alzheimer's Society. With a weakened immune system, the person can become more susceptible to potentially life-threatening infections, according to the National Institute on Aging.

These effects on the body are most pronounced in the advanced stages of the disease, which lasts about 1.5 to 2 years, on average, according to the National Institutes of Health.

Deaths from Alzheimer's disease are on the rise in the United States, according to a recent report from the Centers for Disease Control and Prevention (CDC). The report found that the rate of death from Alzheimer's disease increased by more than 50 percent, from about 16 deaths per 100,000 people in 1999 to 25 deaths per 100,000 people in 2014.

The number of people who died from Alzheimer's also more than doubled during this period, from 44,536 deaths in 1999 to 93,541 deaths in 2014.

The increasing number of deaths from Alzheimer's is partly due to the growing number of older adults in the United States, as Alzheimer's disease most commonly affects adults ages 65 years and older, the CDC researchers said.

But the rise in the rate of deaths from the disease may also be due to an increase among doctors, coroners and medical examiners specifically reporting Alzheimer's disease as a cause of death, the report said.

Still, overall, researchers may underestimate the true number of Alzheimer's deaths, since some doctors may report pneumonia or other complications as the cause of death, rather than Alzheimer's itself. A 2014 study estimated that as many as 500,000 people in the United States died from Alzheimer's in 2010.

Alzheimer's disease is the sixth-leading cause of death in the United States, according to the CDC.

Editor's note: Portions of this article were previously published on Live Science.

Original article on Live Science.