Alzheimer's disease, a devastating neuro-degenerative condition that causes dementia, affects more than 30 million people worldwide and remains an illness for which there is no cure.

For World Alzheimer's Day today, here are five things to know about an illness that was brought to the world's attention by afflicting personalities such as late US president Ronald Reagan and British fantasy author Terry Pratchett.

What is Alzheimer's? 

German doctor Alois Alzheimer was credited with recognizing the disease in 1906, first spotting the plaques and tangles in the brain of a dementia patient who died.

Alzheimer's progressively destroys brain tissue, robbing people of their memory, leaving them disoriented, and at times unable to carry out everyday tasks.

It is also associated with dramatic mood swings and trouble communicating.

The numbers

According to the World Health Organization, more than 55 million people around the world suffer from dementia, of which Alzheimer's is the most common form.

Alzheimer's represents 60 to 70 percent of dementia cases, or more than 30 million.

As life expectancy soars, the prevalence of Alzheimer's in lower- and middle-income countries is climbing. The WHO expects the number of people living with Alzheimer's to triple by 2050.

That will increase the burden on families and on healthcare systems.

Alzheimer's and dementia are among the main causes of problems with mobility and dependency for elderly people.

The causes

Alzheimer's is by far the most common form of dementia, but little is known about what precisely causes it or how it progresses.

In Alzheimer's disease, two key proteins — tau and amyloid-beta — build up into tangles and plaques — known together as aggregates — that cause brain cells to die and lead to brain shrinkage.

Little is known about what causes the proteins to appear or the link between them.

In a new study in 2021, researchers said the toxic protein clusters reach different parts of the brain early on and then accumulate over the course of decades.

Cures

The difficulty in pinpointing the precise cause of Alzheimer's, despite decades of research, has hampered the development of a treatment or cure for the illness.

The biggest advance over the past two decades came in 2021 when the United States approved a drug called Aduhelm, the first new medicine against the disease in almost two decades and the first to address cognitive decline.

However, it has limited effectiveness and medical opinion is divided over its therapeutic benefits.

Risk factors, prevention

According to the French National Institute of Health and Medical Research (Inserm), the main risk factor for Alzheimer's is age: The likelihood of getting it increases after the age of 65, rising steeply after 80.

Diabetes and high blood pressure, when not dealt with at middle age, are also linked to prevalence of the illness, although the medical community has not been able to explain the link.

A sedentary lifestyle is another risk factor, as well as the kind of head traumas suffered by boxers and rugby players.

Studies, stimulating work, and an active social life all seem to delay the appearance of the first symptoms and their severity.

This is because they create a "cognitive reserve" that compensates temporarily for the lost brain cells, reflecting the plasticity of the brain and its capacity to adapt.