Alzheimer’s disease: what it is, what it does to the brain, and how it shows up in daily life

Alzheimer’s disease is the most common cause of dementia, yet for many families it arrives as symptoms long before it arrives as a diagnosis. This first post in our series sets out the basic picture: what the disease is, what it does to the brain, and how those changes show up in everyday life, from the earliest years to the late stages.

Please note: This is an informational post written primarily for carers. It is not diagnostic or treatment guidance, and this series does not discuss medications. For anything specific to you or someone you care for, please consult a specialist.

What this post does not cover: Several related topics have their own dedicated posts in this series: how normal aging, mild cognitive impairment, dementia and Alzheimer’s differ; the other types of dementia and how common they are; risk factors and genes; how Alzheimer’s is detected and diagnosed; the scientific debates about what Alzheimer’s is; and practical care, both for the person and for the people who care for them.

Key points

• Alzheimer’s is a progressive brain disease defined by two abnormal protein deposits, amyloid plaques and tau tangles, together with the loss of connections between nerve cells and of the cells themselves.

• The changes usually begin in memory-related regions deep in the temporal lobe and spread outward over years, which helps explain why learning new information is so often affected first.

• The disease develops slowly: changes in the brain can be present for years before any symptoms appear.

• In daily life it affects memory, planning, language, spatial skills, mood and behavior, and often the person’s awareness of their own difficulties.

• A sudden change in someone with Alzheimer’s is not typical of the disease and is a reason to seek medical help promptly.

What Alzheimer’s disease is

Alzheimer’s disease is a progressive neurodegenerative disease: it gradually damages and destroys nerve cells in the brain, and it worsens over time (Scheltens et al., 2021). It is the most common cause of dementia, the syndrome in which a decline in thinking becomes severe enough to affect independence in everyday life (World Health Organization [WHO], 2025).

The disease is recognized in two ways. In life, clinicians look for a characteristic pattern of symptoms that begins gradually and worsens over time (McKhann et al., 2011). In the brain, it is defined by two kinds of abnormal protein deposits, amyloid plaques and neurofibrillary tangles made of a protein called tau, together with the loss of nerve cells (DeTure & Dickson, 2019).

Alzheimer’s mostly affects older people, and age is its strongest risk factor (Hou et al., 2019). When symptoms begin before 65, it is called early-onset or young-onset Alzheimer’s. These younger cases are more likely to begin with problems other than memory, which we return to below (Graff-Radford et al., 2021).

What happens in the brain

Two abnormal proteins

Amyloid-β is a small protein fragment cut from a larger protein that nerve cells make. In Alzheimer’s, it builds up in the spaces between nerve cells and clumps into plaques. Tau normally helps keep the internal support structure of nerve cells stable. In Alzheimer’s, it changes chemically, comes loose and twists into tangles inside the cells (DeTure & Dickson, 2019; Scheltens et al., 2021).

How much each of these proteins drives the disease is still debated, and we look at those arguments in a dedicated post later in this series.

Losing connections, then cells

Nerve cells communicate through connections called synapses. In Alzheimer’s, synapses are lost, and then the cells themselves die. A classic study comparing brain tissue with people’s cognitive test scores found that synapse loss was the change most closely linked to how impaired they were (Terry et al., 1991). A later review found that, of the two hallmark deposits, the amount of tau tangles in the outer layer of the brain, the neocortex, tracked the severity of cognitive impairment more closely than amyloid plaques did (Nelson et al., 2012).

Where it starts and how it spreads

Tangles tend to appear in a fairly predictable order. Examining brains at different stages, the neuropathologists Heiko and Eva Braak described changes that begin in a small region next to the hippocampus, then spread into the hippocampus and nearby structures of the limbic system, and only later reach most of the outer cortex (Braak & Braak, 1991). The hippocampus is thought to be central to forming new memories, which helps explain why learning and recalling new information is so often the first thing to change.

As the disease advances, the brain visibly shrinks, especially the hippocampus and the temporal lobes, and the brain’s own immune cells become activated; inflammation is now widely studied as part of the disease process (Scheltens et al., 2021). Many people with Alzheimer’s also have other changes in the brain, such as damage to small blood vessels (Schneider et al., 2009). Our post on the types of dementia looks at how these changes combine.

Before symptoms appear: three phases

Alzheimer’s does not begin when symptoms begin. Diagnostic frameworks describe a continuum with three broad phases (Albert et al., 2011; McKhann et al., 2011; Sperling et al., 2011):

• A preclinical phase: brain changes build up without noticeable symptoms, possibly for years or even decades.

• Mild cognitive impairment: changes in thinking are noticeable, but the person largely keeps their independence in daily life.

• Dementia: the changes interfere with everyday activities.

Not everyone with these brain changes moves through all three phases. Our post on normal aging, mild cognitive impairment and dementia looks at how often each path occurs.

How it shows up in daily life

The changes below are common, but no two people follow the same pattern, and many of them can have other causes. They describe the disease; they are not a checklist for diagnosis.

Memory

Trouble learning and recalling new information is the most common first symptom (McKhann et al., 2011). In daily life it can look like asking the same question again, forgetting a recent conversation or appointment, or misplacing things. Older memories are often thought to hold up longer than recent ones, which can puzzle families: someone may describe their childhood clearly but not remember this morning.

Planning, judgment and complex tasks

Reasoning and the handling of complex tasks can be affected (McKhann et al., 2011): managing money and bills, following a recipe, keeping track of appointments, or making decisions that used to be routine. Complex activities like these are typically among the first everyday tasks to need help (Reisberg, 1988).

Language

Finding words becomes harder (McKhann et al., 2011). Conversations may include more pauses, a vaguer word in place of a precise one, or losing the thread partway through.

Spatial skills and getting around

Problems with visual and spatial processing can make it harder to recognize faces or objects, judge distances, or find the way, even in familiar places (McKhann et al., 2011).

Mood and behavior

Changes in mood and behavior are common in Alzheimer’s and are considered part of the disease. A meta-analysis of 48 studies estimated that about half of people with Alzheimer’s show apathy, a loss of interest and initiative; around 40% show depression, anxiety, aggression or sleep problems; about a third have delusions, fixed false beliefs such as being convinced that something has been stolen; and about one in six have hallucinations (Zhao et al., 2016). The estimates varied widely between studies, and how common some symptoms were depended on factors such as the severity of cognitive impairment (Zhao et al., 2016).

Awareness of one’s own difficulties

Many people with Alzheimer’s are less aware of their difficulties than the people around them are, a feature called anosognosia. It is present in a large proportion of people even at the mild stage and becomes more common as the disease progresses. It is linked to greater burden for carers and to risky behavior, for example continuing activities that are no longer safe (Starkstein, 2014). Because it is often part of the disease rather than denial or stubbornness, it changes how families may need to approach conversations about help.

When memory is not the first change

In some people, especially those whose symptoms start before 65, the first problems are with vision and spatial skills, language, or planning and behavior, rather than memory. In clinic-based studies, these atypical presentations made up about a third of young-onset cases, compared with about 6% of later-onset cases, and in one study of young-onset cases, they were far more often misdiagnosed at first (Graff-Radford et al., 2021).

How it changes over time

Clinicians use rating scales to describe how far the disease has progressed in everyday terms. The Clinical Dementia Rating, for example, rates memory, orientation, judgment, community affairs, home life and hobbies, and personal care (Morris, 1993). The Functional Assessment Staging scale describes the order in which everyday abilities tend to be lost (Reisberg, 1988). In broad strokes:

• Mild dementia: complex activities become difficult, such as managing finances, planning a meal for guests or shopping (Reisberg, 1988).

• Moderate to moderately severe dementia: help is needed choosing suitable clothes for the day or season, and later with dressing, bathing and using the toilet; incontinence may begin (Reisberg, 1988).

• Severe dementia: speech becomes very limited, and the person gradually loses the ability to walk and then to sit up without support (Reisberg, 1988).

The pace varies a great deal from person to person, and not everyone follows this order. Stages describe a typical course; they do not set a timetable for any individual.

When it might be something else

A caution: This section is not a guide to telling conditions apart, and it cannot replace a clinical assessment.

Alzheimer’s usually changes slowly, over months and years. A sudden change, over hours or days, in someone’s attention, alertness or confusion is different. It may be delirium, an acute state of confusion that starts suddenly and comes and goes, often triggered by an infection, a medication, dehydration or another medical problem (Inouye et al., 2014). Dementia is a leading risk factor for delirium, the two often occur together, and in people with dementia delirium is linked to faster decline and higher mortality (Inouye et al., 2014). A sudden change is therefore a reason to seek medical help promptly, not something to put down to the disease.

Other conditions, such as depression and some medical problems, can also affect memory and thinking or make Alzheimer’s symptoms worse, and a few contributing causes are treatable (Clarfield, 2003). This is one more reason a specialist assessment matters.

What this means for carers

Understanding what the disease does can make some changes easier to interpret. Repeated questions or reduced awareness of difficulties are symptoms of the disease, not inattention or denial. In the early stages, many people compensate for their difficulties, often without realizing it: they find workarounds, lean on routines, or quietly avoid tasks that have become hard. This ability of the brain to cope with damage for a while is useful (Stern, 2012), but it can also hide changes from the person and from those around them. Knowing the usual course of the disease can help families make sense of what they see, and decide when to seek an assessment.

What you notice is also valuable to clinicians: what changed, roughly when, whether it came on gradually or suddenly, and how it affects daily life. Assessment typically draws on a history from someone who knows the person well (McKhann et al., 2011).

Later posts in this series turn to practical care: communicating with the person, daily life and safety, and looking after the people who care. Questions about treatment belong with the person’s doctor. If you are worried about your own memory or someone else’s, a doctor experienced in cognitive assessment is the right place to start.