Soaring rates of population growth and ageing have long been seen as portending a global explosion of Alzheimer's, the debilitating disease that robs older people of their memory and independence.
But an unexpected, and hopeful, trend may be emerging.
In rich countries at least, recent data suggests the rate of new cases has slowed or even reversed -- a tantalizing hint that quality-of-life improvements may protect against dementia.
"These findings are promising, and suggest that identifying and reducing risk factors for Alzheimer's and other dementias may be effective," Keith Fargo, scientific director at the , told AFP.
Overall numbers will keep growing for now -- albeit at a slower rate -- as more and more people live ever longer, he noted.
"We have stemmed the flow, but we haven't stopped it," added David Reynolds of .
According to the , dementia affects some 47.5 million people worldwide -- with 7.7 million new cases every year.
Alzheimer's is the most common cause, responsible for 60-70 percent of dementia cases.
The disease, which claimed actor Gene Wilder last month, typically progresses from forgetfulness and absent-mindedness to major memory loss and near total dependence as sufferers become unaware of time and place.
Towards the end, those afflicted can forget how to eat.
Alzheimer's was first identified more than 100 years ago, but there is still no effective treatment or cure, and scientists disagree on its causes.
A main culprit is thought to be the buildup of protein plaques on the brain.
Some recent studies have linked the condition to air pollution, fungus or even accidental transmission during a medical procedure.
- Use it or lose it -
New studies pointing to an Alzheimer's slowdown in rich countries, especially among men, imply that a healthy lifestyle -- and plenty of brain exercise -- may slow or stave off dementia.
Such trends have been observed in the United States, the Netherlands, Sweden, and Spain.
Britain had the biggest reversal -- Alzheimer's incidence there dropped 20 percent in as many years.
A study in the journal Nature Communications in April reported 209,000 new cases in Britain in 2015 -- far fewer than the 251,000 forecast in 1991 based on population growth and ageing trends.
This meant the likelihood for British over-65s of developing dementia was "lower than it was for the previous generation", the authors concluded.
The reasons are not clear.
Some researchers point to improved cardiovascular health stemming from a growing awareness of the dangers of smoking, obesity and a lack of exercise.
Better high blood pressure and cholesterol drugs may also play a role.
Several studies have linked brain stimulation to lower dementia risk -- whether in the form of high-level schooling, a cerebrally-challenging job, or simply filling out a crossword or Sudoku.
"It's the old adage of use it or lose it," Reynolds said.
Further research is needed to prove that these factors act as dementia shields.
In the meantime, public health policy should encourage "better environments and healthier societies", said Carol Brayne of the Cambridge Institute of Public Health.
This is no time for complacency, she and other experts said ahead of World Alzheimer's Day.
In developing countries, incidence rates may be underestimated and are likely to rise as medical care improves and more people make it to their 80s.
"There are other things that have changed that may push it (the trend) in the wrong direction," said Reynolds.
"Diabetes and obesity have been rising rapidly over the last 20 years," he added.
"So it is possible that whilst we are in many ways healthier, for other reasons we have made ourselves less healthy... and that may then either reduce the decline or even push up the rates of dementia."
