BBC's Rob Watson: "of the 30m people... infected with the HIV virus 86% live in the... sub-Saharan Africa"

Worldwide the situation looks more optimistic but the UN says the effect of Aids has forced it to cut its long-term projection for the world's population by 400m.

The new report on population trends says the vast majority of the 30 million people in the world currently infected with the HIV virus live in the countries of sub-Saharan Africa.

The head of the UN's Population Division, Joseph Chamie, said the continuing spread of the virus through the continent would have a devastating effect on life expectancy there.

"There'll be a heavy, heavy death toll relating to Aids," he said.

Life expectancy - 47 and shrinking

On average, in the 29 hardest hit African countries, people live for seven years less due to Aids - life expectancy at birth is now estimated at 47 years.

But in, Botswana, the worst affected country where one of every four adults is infected, life expectancy is anticipated to fall further, to 41 years by 2005.

Other countries named in the report include Botswana, Kenya, Malawi, Mozambique, Namibia, Rwanda, South Africa, Zambia and Zimbabwe, where more than 10% of the adult population is infected by the HIV virus

Eventually, in some African countries, the average life expectancy may drop by 20 years.

Optimism out of Africa

Outside Africa, though, the report is more optimistic, predicting that in 2050 many people will live for one hundred years or more.

In 1999 there will be 6bn of us

Joseph Chamie said: "Lower death rates and longer life in most countries is probably humanity's greatest achievement.

"From biblical times there has been this attempt to live a long, healthy life - and that's what's been achieved now," he said.

The world according to the UN is ageing, with an ever growing percentage of the population being over 60 years old.

Europe is most affected, with the proportion of older people expected to increase from 20% in 1998 to 35% in 2050.

World population to reach nine billion

The UN says it expects the world's population to increase from 5.9 to almost nine billion people by the middle of next century but the growth rate is decreasing.