I thought I already knew all I needed to about the impact of HIV and AIDS in Africa.

Watch the Newsnight report by Evan Davis

Certainly I knew it was a human catastrophe, that the virus was running through populations on a scale unknown in the West, in an area of the world ill-equipped to cope.

But it was only when a British businessman with extensive experience in Zimbabwe described to me some of the practical effects the illness is having on society there, that I decided I should find out more.

Living in the shadow of Aids

Zimbabwe now has the dubious honour of being the world's most infected country - about a quarter of the adult population is HIV positive. In many urban areas, infection runs to 40%. In the army, it is more like 80%.

Life expectancy at birth, on one estimate, is poised to fall to 38 years. The country suffers from having an economy advanced enough for the virus to spread, in particular, on relatively good roads - epidemiologists have tracked high HIV prevalence along the main freight routes.

Alas, the economy may be strong enough to help HIV, but it is not strong enough to fight it. Zimbabweans cannot get modern anti-retroviral drug combinations because in Zimbabwe, the annual health budget is about �5.50 per person, enough to finance a modern drug regime for about five hours a year.

Orphaned by Aids: Life expectancy at birth on one estimate is poised to fall to 38%

The effect of HIV on the country mirrors its effect on individuals. In the absence of treatment, a typical HIV patient has a relatively normal life for six for seven years in Africa, before succumbing to life-threatening AIDS symptoms.

So it has been with Zimbabwe - HIV infection spread rapidly between 1991 and 1996 - but the country was able to function for several few years before HIV-related deaths started occurring in earnest. But that gruesome new phase is now underway, with profound effects.

Grim turning point

We visited a large cemetery outside Harare - one of the guards told us that the rate bodies arrived had increased markedly in December 1997 - "too much nightclubbing," he explained.

Now, across the country, among a population about twice the size of that in London, an HIV death occurs every few minutes. Indeed, in some areas, HIV appears to have reached a grim turning point, in that the number of deaths is outstripping the number of new infections, taking the overall level of HIV prevalence down.

Mourning: HIV infection spread rapidly between 1991 and 1996

The human effect of the disease is obvious, the social and economic effects subtle, but real.

Although the combined effects of stigma, denial, and confusion with other manifestations of poverty make HIV a difficult subject to discuss and analyse in Zimbabwe, it is clear that within households and workplaces, the disease is affecting the ability of Zimbabwe to look to and think about its future.

For example, school attendance suffers, as sick parents cannot afford school fees, or require their children to look after them at home.

Pay-outs threat

Saving and investment suffer, as households burn up their slim reserves to survive in the absence of a breadwinner.

The financial sector malfunctions - as it attempts to protect itself from the potentially crippling impact of HIV-related pay-outs for the sick and disabled, by preventing individuals from buying a significant amount of life insurance. Negative HIV tests are pre-requisites for significant cover, but 80% of those wanting life insurance decline to take the test.

Grave digging: In many urban areas, infection runs to 40%

In the workplace, costs have risen, with absenteeism, higher staff turnover, and less incentive to train - how can you afford to train staff if they could be ill within months?

In the countryside, the burden of 'returnees' - individuals who migrate to the towns to get work, who get sick and then go back to their village to die - and the catastrophe of the ever-growing number of orphans is forcing social change. In some places, communal land has been set-aside simply to provide for the children.

No one likes to say it, but the impact of HIV is particularly deep because unlike most health disasters to befall Africa, this one is killing the strong, the parents, the productive, the very people on whom functioning societies inevitably rely.

Worst fears

It even sometimes most affects the more skilled and better trained. It was explained to me that they have had more time, energy and cash to spend to get themselves infected.

All in all, in Zimbabwe it is a disaster far worse than anything anyone expected at the time the worst fears of HIV were being discussed.

Zimbabwe is not a society without hope. People cope - there are "a thousand and one coping strategies" one HIV activist told us. Even people who are in denial can learn to cope, and people in Africa are more familiar than most with the need to cope.

But still, the brave attempts of Zimbabweans to live life as normal in the face of a disaster unlike anything we could conceive, is both sad and impressive.