Alzheimer’s disease is a progressive brain disorder that slowly destroys memory, thinking skills, and eventually the ability to carry out basic daily tasks. It is the most common form of dementia, contributing to 60 to 70 percent of all dementia cases worldwide. In 2021, an estimated 57 million people were living with dementia globally, with Alzheimer’s accounting for the majority. The disease typically develops slowly over many years, and most people live 4 to 8 years after diagnosis, though some live much longer depending on age and overall health.
What Happens in the Brain
Alzheimer’s centers on two proteins that gradually damage and kill brain cells. The first is beta-amyloid, a fragment of a larger protein. When these fragments clump together, they form sticky deposits called amyloid plaques that build up between brain cells and disrupt their ability to communicate with each other.
The second protein is tau. Normally, tau helps maintain the internal transport system that carries nutrients through brain cells. In Alzheimer’s, tau proteins change shape and twist into structures called neurofibrillary tangles. These tangles collapse the transport system from within, starving cells of what they need to function. Over time, affected brain cells die, and the brain physically shrinks. This damage starts in areas responsible for memory and gradually spreads to regions that control language, reasoning, and eventually basic body functions like swallowing and breathing.
Early Warning Signs vs. Normal Aging
Everyone forgets things occasionally, and not all memory lapses signal Alzheimer’s. The key difference is in frequency, severity, and whether you can eventually recall what you forgot. Making a bad decision once in a while is normal aging. Making poor judgments and decisions repeatedly is a warning sign. Missing a monthly payment happens to anyone. Consistently struggling to manage bills is different. Forgetting which day it is and remembering later is ordinary. Losing track of the date or season is not.
Other early signs include trouble following or joining conversations (beyond occasionally struggling to find the right word), misplacing things frequently and being unable to retrace your steps to find them, and difficulty completing familiar tasks that used to be routine. People in early stages may also need more time to finish things they’ve done their whole lives, and personality shifts like unusual withdrawal from social situations or uncharacteristic irritability can appear before significant memory loss does.
How the Disease Progresses
Alzheimer’s moves through a preclinical phase, then mild cognitive impairment, and finally three recognized stages of dementia: mild, moderate, and severe. The preclinical phase can last years or even decades, with changes occurring in the brain long before anyone notices symptoms.
Mild Stage
People experience growing memory loss alongside other cognitive difficulties. Wandering and getting lost become more common, even in familiar places. Handling money, paying bills, and repeating questions are frequent struggles. Planning a family event or balancing a checkbook can feel overwhelming. Many people show lapses in judgment, particularly around financial decisions, and personality changes like becoming quieter, more withdrawn, or unexpectedly irritable start to emerge. Most people can still live independently at this stage, but they need increasing support.
Moderate Stage
This is typically the longest stage and the one where caregiving demands increase dramatically. Damage spreads to brain areas that control language, reasoning, and sensory processing. People begin having trouble recognizing family and friends. They may be unable to learn new information, carry out multistep tasks like getting dressed, or cope with unfamiliar situations. Hallucinations, delusions, paranoia, and impulsive behavior can appear during this stage. Wandering becomes more dangerous, and most people need daily supervision and assistance.
Severe Stage
By this point, plaques and tangles have spread throughout the brain, and brain tissue has shrunk significantly. People lose the ability to communicate and become completely dependent on others for all care. Near the end of life, the person may spend most or all of their time in bed as the body gradually shuts down. Death typically results from complications like infections rather than from Alzheimer’s itself.
Who Is at Risk
Age is the single strongest risk factor. The vast majority of people with Alzheimer’s are 65 or older, and risk roughly doubles every five years after that. Less than 10 percent of all Alzheimer’s cases are early-onset, meaning symptoms appear before age 65. Early-onset cases are usually driven by inherited gene mutations that make the disease almost certain to develop, sometimes well before middle age.
For the more common late-onset form, genetics still plays a role but doesn’t determine your fate. The most well-known genetic risk factor is a variant of the APOE gene called APOE e4. About 15 to 25 percent of people carry one copy of this variant, and 2 to 5 percent carry two copies. Having two copies raises risk significantly more than having one. But carrying the variant isn’t a guarantee: some people with APOE e4 never develop Alzheimer’s, and many people who develop the disease don’t carry it at all.
Lifestyle and environmental factors also shape risk, sometimes by affecting how genes behave. Cardiovascular health matters considerably. Conditions like high blood pressure, diabetes, obesity, and smoking all increase the likelihood of developing Alzheimer’s. On the protective side, regular physical exercise, social engagement, cognitive stimulation, and a heart-healthy diet are associated with lower risk. Hearing loss has also been identified as a modifiable risk factor, likely because untreated hearing loss reduces social and cognitive engagement over time.
How Alzheimer’s Is Diagnosed
There is no single test for Alzheimer’s. Diagnosis involves a combination of cognitive assessments, medical history, and increasingly, biological testing to detect the disease’s signature proteins.
Brain imaging can reveal whether someone has high levels of amyloid plaques. Normal amyloid levels on a scan suggest that Alzheimer’s is not the cause of someone’s symptoms. Cerebrospinal fluid tests, which involve a spinal tap, can detect abnormal levels of both amyloid and tau proteins, sometimes in the early stages before significant symptoms appear. Several of these CSF tests have received FDA approval and are now in clinical use.
Blood-based tests are a newer and less invasive option. These tests measure blood levels of tau, amyloid, and other markers associated with Alzheimer’s. Some are already being used in doctors’ offices, and in mid-2025, the Alzheimer’s Association published formal clinical guidelines on using blood-based biomarker testing to diagnose the disease in specialty care settings. This is a meaningful shift, as blood tests could eventually make early diagnosis far more accessible than brain scans or spinal taps.
Current Treatment Options
For decades, the only medications available for Alzheimer’s managed symptoms without affecting the underlying disease. Those drugs, which work by boosting chemical messengers between brain cells, can temporarily improve memory and thinking in some people but do not slow the progression of brain damage.
A newer class of treatments targets the disease itself. These are antibody-based infusions designed to clear amyloid plaques from the brain. The FDA has approved donanemab (sold as Kisunla) for adults with early-stage Alzheimer’s who have confirmed amyloid buildup. In clinical trials involving over 1,700 patients, those receiving the treatment showed a statistically significant reduction in cognitive and functional decline compared to those on placebo over about 18 months. The treatment is given as an intravenous infusion every four weeks.
These newer treatments are not cures. They slow the rate of decline rather than stopping or reversing it, and they carry risks including brain swelling and small brain bleeds that require monitoring with regular brain scans. They are also only approved for people in the mild cognitive impairment or mild dementia stage, making early diagnosis more important than ever. For people in moderate or severe stages, treatment focuses on managing symptoms, maintaining comfort, and supporting caregivers.