A mother and daughter living with Alzheimers.

What is Alzheimer's disease?

Alzheimer’s disease is a brain disorder that cannot be stopped or reversed. The disease severely affects memory, thinking, learning and organizing skills and eventually affects a person’s ability to carry out simple daily activities. Alzheimer’s disease is not a normal part of the aging process.

Alzheimer’s is a disease whose symptoms worsen over time. In fact, scientists believe the disease process may go on for 10 years or longer before the first symptoms of Alzheimer’s disease appear.

When memory problems do begin to be noticeable, they are often identified as mild cognitive impairment (MCI). At this stage, intellectual function is affected but the ability to function and live independently remain intact as the brain compensates for disease-related changes.

In some people, MCI can hold steady at this stage. However, people with MCI are at high risk for progressing to dementia. Alzheimer’s disease is the most common form of dementia. (Dementia can also be due to a variety of reasons such as Parkinson’s disease, dementia with Lewy bodies, vascular dementia, frontotemporal dementia and more.) With dementia, in contrast to MCI, daily function is affected.

As dementia due to Alzheimer’s disease progresses to late stages, affected individuals cannot carry on a conversation, recognize family and friends, or care for themselves.

Normal age-related memory changes vs memory changes in Alzheimer's disease

How common is Alzheimer’s disease?

Alzheimer’s disease is the most common cause of dementia (accounting for 60 percent to 80 percent of cases). Alzheimer’s disease is the sixth leading cause of death in the United States.

One in 10 people older than 65 and nearly half of people older than 85 have Alzheimer’s disease. Alzheimer’s disease can also affect people in their 40s. The percentage of people who have Alzheimer’s disease rises every decade beyond the age of 60. According to the Alzheimer's Association, with the aging of the population and without successful treatment, there will be 14 million Americans and 106 million people worldwide with Alzheimer’s disease by 2050.

What causes Alzheimer’s disease?

Alzheimer’s disease is caused by the abnormal build-up of proteins in the brain. The build-up of these proteins — called amyloid protein and tau protein — leads to cell death.

The human brain contains over 100 billion nerve cells as well as other cells. The nerve cells work together to fulfill all the communications needed to perform such functions as thinking, learning, remembering, and planning. Scientists believe that amyloid protein builds up in the brain cells, forming larger masses called plaques. Twisted fibers of another protein called tau form into tangles. These plaques and tangles block the communication between nerve cells, which prevents them from carrying out their processes. The slow and ongoing death of the nerve cells, starting in one area of the brain (usually in the area of the brain that controls memory) then spreading to other areas, results in the symptoms seen in patients with Alzheimer’s disease.

What are the symptoms of Alzheimer’s disease?

Symptoms of Alzheimer’s disease vary from person to person and worsen over time. Symptoms of the disease include:

  • Memory loss. This is usually one of the first symptoms of Alzheimer’s disease.
  • Putting objects in odd places
  • Confusion about events, time and place
  • Repeating questions
  • Trouble managing money and paying bills
  • Trouble performing/taking longer to perform familiar tasks
  • Getting lost/wandering
  • Not being able to sleep
  • Changes in personality and behavior including agitation, anxiety and aggression
  • Having groundless suspicions about family, friends and caregivers
  • Poor judgment or reasoning
  • Trouble recognizing family and friends
  • Difficulty learning and remembering new information/recent events
  • Difficulty performing multistep tasks, such as dressing or cooking
  • Having hallucinations, delusions or paranoia
  • Difficulty speaking/finding the right words
  • Difficulty reading, writing and working with numbers
  • Difficulty walking
  • Difficulty swallowing

For more information on the stage of disease, click here.

Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy