JOHANNESBURG (Reuters) - Infectious diseases shaped the course of European history, shaking the foundations of imperial Rome and bringing the medieval world to its knees.

They continue their grim march through the ages, with HIV/AIDS reaping death and despair on a vast scale in Africa.

"Contagious diseases have had a very decisive impact on history and are right now very evidently having a huge impact on Africa," Professor Howard Phillips, a medical historian at the University of Cape Town, told Reuters.

Africa's HIV/AIDS crisis will be high on the agenda of the UN summit opening in Johannesburg on August 26. World leaders will try to map out a global strategy for eradicating poverty and disease without inflicting irreparable harm on the planet.

The World Summit on Sustainable Development will try to agree on a strategy for achieving the UN Millennium goals, which include halting the spread of AIDS by 2015.

As the delegates confront the horror of the pandemic in Africa, they could do worse than study the great plagues of the past for clues on how the continent's unfolding biomedical tragedy could mold its future.

Sub-Saharan Africa is the epicenter of the global HIV/AIDS pandemic with more than 28 million of the world's 40 million infected people, according to UN estimates. In the continent's southern cone, infection rates in some countries are in excess of 30%.

US government researchers say the average life expectancy of people in 11 African countries will drop below 40 by 2010 as the disease shortens the lives of millions.

And if history is anything to go by, Africa is in more trouble than even these figures suggest.

PLAGUES PAST AND PRESENT

Plagues have littered the past with the corpses of countless victims. The Roman Empire, stretching from the deserts of southern Egypt to the rugged hills of Scotland, succumbed.

One school of thought holds that waves of infectious diseases--including smallpox, gonorrhea and bubonic plague--ravaged the Mediterranean world between AD 250 and AD 650, contributing to Rome's decline.

The historian Norman F. Cantor in his book, "In the Wake of the Plague," writes that this assault of germs "reduced the human population (of the Roman world) by at least one-quarter."

This created a manpower shortage in a society whose productivity depended almost entirely on plentiful human labor.

"The result was far-reaching: a decline in food supplies and drastic reduction in industrial production," says Cantor.

"This caused shrinking of an already inadequate tax base, decreasing funds for bureaucracy and defense."

AIDS is having a similar impact on Africa.

By some estimates, over 25% of the workforce may be lost to AIDS by 2020 in some badly hit African countries.

Agricultural productivity is in decline in many southern African countries as laborers become too sick to work the land, exacerbating food shortages caused by drought and politics.

In South Africa, where one in nine people is infected, the economy could be 1.5% lower by 2010 than it would be without AIDS and 5.7% lower by 2015, says one study by the Bureau for Economic Research, an independent think tank.

As in Rome, taxes, the skills base and production are all eroding, eating away at both the economy and the state.

MEDIEVAL CATASTROPHE

The biomedical crisis most etched in the West's mind is bubonic plague or so-called "Black Death."

Carried by parasites on the backs of rodents, it killed between one-third and one-half of Europe's population from 1347 to 1350. World War II, which raged for six years, did not claim such a high proportion of lives on the continent.

According to Cantor, the English and Welsh population of close to six million in 1300 was not reached again until the mid-18th century.

The effects of such a catastrophe were dire.

"It (the black death) deepened antagonism between rich and poor and raised the level of human hostility," writes Barbara W. Tuchman in her work "A Distant Mirror."

Jews, accused of poisoning wells, were targeted as scapegoats in savage pogroms.

"As in all primarily rural societies during times of economic upheaval, there was a flocking of 'misdoers'--criminals, beggars, and prostitutes--to London from the countryside. It was like many parts of sub-Saharan Africa and Latin America today," writes Cantor.

ENTER AIDS

AIDS may prove to have more profound consequences on modern Africa than the plague did on Europe.

For a start, the plague snuffed out rich and poor, young and old alike--although the ratio of old and very young victims was probably higher than that of those in their prime.

AIDS is having the opposite demographic effect, taking out a disproportionate number of breadwinners and people of working age--most of its victims are between the ages of 15 and 49--and leaving behind an army of orphans and other dependents.

The UN estimates that almost 6% of all children in Africa will be orphaned by AIDS by 2010. By last year alone, more than 10 million children in sub-Saharan Africa are believed to have lost one or both parents to the pandemic.

Security analysts say this is fueling crime on African streets and creating a vast supply of recruits for the continent's many armies and rag-tag rebel groups that rely heavily on child soldiers to fight their battles.

The crisis is clearly deepening poverty as it robs families of breadwinners, adding to the tensions between the continent's many have-nots and few but well-heeled haves--not unlike medieval Europe, but on a far grander scale.

Xenophobia and violence against migrants accused of stealing scarce jobs may well be exacerbated by this crisis-induced poverty in a chilling echo of the medieval persecution of Jews.

The light at the end of this tunnel is the wonder of modern medicine: medieval man looked to the stars and the supernatural to explain the inexplicable. We know the cause of AIDS--HIV--and the reasons behind its spread.

Africa has the advantage of strong communal bonds, especially in its rural areas, which help families to cope.

If some African countries want to avoid the fate of the Roman Empire, drastic action will be required on the part of the continent's governments and western donors to stem the pandemic

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