India, China and countries in Latin America will be the worst hit, a conference heard.

Dr Martin Price, of the Institute of Psychiatry in London, presented the figure at a gathering of Alzheimer's disease specialists in India.

Evidence

He told the 14th annual conference of Alzheimer's Disease International on Friday: "Studies show that more than three-quarters of the people affected by Alzheimer's by the year 2020 would be in developing countries.

Alzheimer's mainly affects the elderly

"This is due to the rapid increase in the population of the elderly in these countries."

Alzheimer's is a degenerative disease of the brain, which destroys memory and eventually all other mental functions.

It mainly affects the elderly but people in their 40s or 50s are known to have developed it.

The disease was becoming more common in many countries, Dr Price said, but China was showing the greatest increase.

The next highest growth in incidence of the disease was in Latin American countries and then India.

Economic implications

The ADI, an umbrella organisation of national Alzheimer's organisations around the world, estimates that there are 18 million cases of the disease worldwide.

This is set to rise to 30 million by 2020.

Professor R. Santhianathan, a psychiatry professor at Madras Medical College, said that it is difficult to study the prevalence of the disease in developing countries.