What Is Alzheimer’s Disease?

Alzheimer’s disease is a progressive neurological disease that damages the brain and gradually affects memory, thinking, behavior, and a person’s ability to carry out everyday activities.

It is the most common cause of dementia. The disease usually affects older adults, although younger-onset Alzheimer’s can occur before age 65. The National Institute on Aging says that most people with Alzheimer’s develop the disease after age 65, while fewer than 10% of cases occur before that age. (NIA: Alzheimer’s Disease Fact Sheet)

Alzheimer’s does not begin with memory loss in every person. For some people, early changes may involve finding the right words, reasoning, judgment, or visual and spatial abilities. Over time, however, the disease can affect many areas of a person’s life. (NIA: Alzheimer’s Disease Fact Sheet)

About 120 years ago, Dr. Alois Alzheimer, a German psychiatrist and neuropathologist, studied a woman named Auguste Deter who developed severe memory problems and other changes in her 50s.

After her death in 1906, Alzheimer examined her brain and described unusual changes that included amyloid plaques and neurofibrillary tangles. His findings became an important part of the history of Alzheimer’s disease. (NIH: The Discovery Of Alzheimer’s Disease)

Today, scientists know much more about these brain changes, but there is still much to learn about exactly how Alzheimer’s disease begins and progresses.

Celebrities such as Ronald Reagan, Charles Bronson, and Rita Hayworth have also been publicly associated with Alzheimer’s disease, helping bring greater public attention to the condition.

Plaques And Tangles

Dr. Alzheimer noticed two unusual structures in the brain of Auguste Deter.

One of these structures was a sticky deposit that formed outside and around nerve cells. These are known as amyloid plaques. The other consisted of abnormal twisted fibers inside nerve cells. These are called neurofibrillary tangles and are made primarily from a protein called tau. (NIA: What Happens To The Brain In Alzheimer’s Disease?)

These plaques and tangles are important biological features of Alzheimer’s disease. They are associated with damage to neurons and the connections between them, which contributes to the progressive problems with memory and thinking seen in Alzheimer’s. (NIH: Diagnosing Dementia)

However, I would not say that plaques and tangles are simply “the main cause” of Alzheimer’s disease.

Scientists now understand that Alzheimer’s is more complicated than that. Amyloid and tau are major parts of the disease process, but other factors, including inflammation, blood vessel changes, genetics, and other forms of brain damage, may also contribute. (NIA: Alzheimer’s Disease Fact Sheet)

There is another interesting point.

Some older people have amyloid plaques and other Alzheimer’s-related brain changes but never develop noticeable Alzheimer’s symptoms. This is one reason scientists continue to study how different brain changes interact with each other. (NIA: Inside The Brain: The Role Of Neuropathology In Alzheimer’s Disease Research)

Common Symptoms Of Alzheimer’s Disease

Memory loss is one of the most familiar signs of Alzheimer’s disease, but it is not the only symptom.

A person may repeatedly ask the same question, forget recent conversations, lose track of appointments, misplace familiar objects, or have trouble remembering newly learned information.

They may also struggle with tasks that involve several steps.

For example, a person who previously handled laundry, cooking, shopping, or paying bills without difficulty may gradually have trouble organizing these activities.

Alzheimer’s can also affect language, judgment, reasoning, and visual-spatial abilities. A person may have trouble finding the right word, following a conversation, making decisions, recognizing familiar surroundings, or judging distance. (NIA: Alzheimer’s Disease Fact Sheet)

Changes in personality and behavior can occur as well.

Someone who was previously calm may become anxious, irritable, suspicious, withdrawn, or unusually restless. These changes are not necessarily deliberate. They can happen because the disease is affecting parts of the brain involved in emotion, judgment, communication, and behavior.

This is why I think caregivers need to look beyond the behavior itself.

If your loved one is acting differently, try to understand what may be happening in their brain rather than simply assuming they are being difficult.

The Root Cause Of Alzheimer’s Disease

Scientists still have not identified one single cause that explains why every person develops Alzheimer’s disease.

Instead, researchers believe that Alzheimer’s results from a combination of factors. These can include aging-related changes in the brain, genetics, environmental influences, and lifestyle or health factors. The importance of each factor can vary from one person to another. (NIA: Alzheimer’s Disease Fact Sheet)

Age is the biggest known risk factor.

Most people with Alzheimer’s are 65 or older, and the risk increases substantially as people get older. However, getting older does not mean that a person is destined to develop Alzheimer’s disease. (NIA: Thinking About Your Risk For Alzheimer’s Disease)

Family history also matters.

Having a parent or sibling with Alzheimer’s can increase a person’s risk, but it does not mean that the person will definitely develop the disease.

There are also rare forms of inherited Alzheimer’s disease.

Scientists have identified rare disease-causing variants in three genes: APP, PSEN1, and PSEN2. These variants can cause early-onset Alzheimer’s disease, often before age 65. When one of these disease-causing variants is present in a parent, each child has a 50% chance of inheriting it. (NIA: What Causes Alzheimer’s Disease?)

This is different from carrying APOE ε4, which is a risk factor rather than a guarantee that someone will develop Alzheimer’s.

How Many People Have Alzheimer’s Disease?

Alzheimer’s disease affects millions of people in the United States.

According to the Alzheimer’s Association’s 2026 Alzheimer’s Disease Facts and Figures report, an estimated 7.4 million Americans age 65 and older are living with clinical Alzheimer’s dementia in 2026. About 74% of them are age 75 or older. (Alzheimer’s Association: 2026 Facts And Figures)

The disease also affects some people younger than 65.

Younger-onset Alzheimer’s is less common, but it does happen. The Alzheimer’s Association estimates that around 200,000 Americans between ages 30 and 64 have younger-onset dementia, although population estimates for younger-onset conditions are limited. (Alzheimer’s Association: 2026 Facts And Figures)

So, the old idea that Alzheimer’s disease is something that only affects people in their 70s or 80s is not correct.

It can affect younger adults too.

Alzheimer’s disease is also a major cause of death in the United States. According to the CDC, Alzheimer’s disease was the sixth-leading cause of death in the United States in 2024, with 116,022 deaths recorded. (CDC: Leading Causes Of Death)

Is There A Cure?

Unfortunately, there is still no cure for Alzheimer’s disease.

However, saying that “nothing can be done” would also be incorrect.

There are medications that can temporarily improve or stabilize symptoms in some people. In addition, newer treatments can target amyloid in the brain and slow the rate of cognitive and functional decline in some people with early Alzheimer’s disease.

For example, the FDA gave traditional approval to lecanemab (Leqembi) in 2023 after a confirmatory trial showed clinical benefit. It is used for people in the mild cognitive impairment or mild dementia stages of Alzheimer’s disease who have confirmed amyloid pathology. (FDA: Leqembi Traditional Approval)

The FDA also approved donanemab (Kisunla) in 2024 for people with Alzheimer’s disease. Treatment is intended for people in the mild cognitive impairment or mild dementia stage, the population studied in the clinical trial. (FDA: Kisunla Approval)

These medications are not cures.

They also are not appropriate for everyone. They can have serious risks and require medical evaluation and monitoring.

This is one reason an early and accurate diagnosis can be useful.

Besides medication, non-drug approaches can help make everyday life safer and more comfortable. A predictable routine, appropriate physical activity, meaningful activities, good nutrition, social connection, and adjustments to the home environment can all play a role in dementia care.

Caregivers also need support.

Taking care of someone with Alzheimer’s can affect your emotional health, physical health, employment, finances, and family life. The Alzheimer’s Association estimates that nearly 13 million Americans provide unpaid care for people with Alzheimer’s or other dementias. (Alzheimer’s Association: 2026 Facts And Figures)

So, when I talk about Alzheimer’s disease, I don’t think the focus should be only on the person who receives the diagnosis.

The caregiver needs help too.

Conclusion

Alzheimer’s disease is a progressive brain disease that gradually damages memory, thinking, behavior, and the ability to manage everyday activities.

It is most common in older adults, but younger-onset Alzheimer’s can occur.

The disease is associated with amyloid plaques and tau tangles, but researchers now understand that Alzheimer’s is more complicated than these two brain changes alone. Age, genetics, health, and other biological factors can all play a role.

There is still no cure, but treatment has changed significantly since I first wrote this article in 2019.

Today, doctors have medications that can help manage symptoms, and there are also FDA-approved treatments that can slow disease progression in some people with early Alzheimer’s disease.

That is why I would not look at an Alzheimer’s diagnosis as meaning that nothing can be done.

An early medical evaluation, proper diagnosis, treatment when appropriate, and good support for both the person with Alzheimer’s and the caregiver can make a meaningful difference in how the disease is managed.