The paths of transmission

Fortunately, our knowledge about the epidemic is growing fast, and with that knowledge comes an appreciation of the many, less intractable, social and biological features that also drive the epidemic. Many poor countries have so far escaped full-blown epidemics (for example the nations of the west African Sahel), while it is middle-income South Africa and Botswana that have been especially hard hit.

A major reason for this difference is the historic patterns of labour migration in southern Africa, a legacy of apartheid and way in which it wrenched apart families. Across the southern tip of the continent, it is commonplace for a man to have a family "at home" in the rural areas, and one or more regular sexual partners in the township or close to his place of work. Where individuals have concurrent sexual partnerships, the virus spreads most rapidly. This is because a person with HIV is most infectious in the few weeks after first becoming infected: it is in this short period of time that the majority of onward transmission occurs.

During the succeeding six or seven years, until the infected individual begins to develop Aids, he or she is hardly infectious at all. Southern Africans don't have more lifetime sexual partners than Europeans or north Americans, but because of this particular pattern of mobility means that they often have concurrent partnerships, the conditions for viral transmission are ideal.

Other key "intermediate" drivers of the epidemic include the absence of male circumcision (again, a factor that places many southern African populations at a disadvantage compared to the Muslim Sahel) and high levels of sexual violence - there is good reason to believe that HIV transmission is more likely when sexual encounters are forcible and violent, and women who are survivors of rape are often socially ostracised and vulnerable, and may find themselves at greater risk of contracting HIV in their subsequent lives.

The effort to tackle HIV/Aids epidemics in southern Africa demands grappling with these particular drivers of the epidemic, as well as continuing efforts at increasing awareness and overcoming stigma and denial.

Most of the world's Aids epidemics are concentrated among high-risk groups, such as commercial sex workers and their clients, injecting drug users, and male homosexuals. The flare-up of generalised epidemics, such as those in southern Africa where a quarter or more of the adult population are living with HIV, has meant that governments and international agencies have tended to target their Aids-prevention messages very broadly, as a pre-emptive measure

It's now becoming clear that the conditions that give rise to a generalised epidemic are relatively rare. That's only small reason for comfort, however. In Russia and China, epidemics among injecting drug users may only affect a small proportion of the overall population, but the numbers are high - and in the case of Russia, already stricken by multiple health crises and a declining population, it is one more factor contributing to social malaise. Concentrated epidemics such as these demand tough and farsighted policy decisions, including promoting harm-reduction programmes such as needle exchanges. China, it appears, is moving in this direction.

A long struggle

Meanwhile, most of Africa's epidemics seem to be stabilising or declining. The best news is that sharp declines in HIV prevalence in Zimbabwe and Kenya are for real. Most likely these reflect improvements in girls' education (notable in Zimbabwe during the 1990s) and greater public awareness of Aids. But there are also reasons for worry. Uganda, for long the poster-child of Aids success - the first country in Africa to register real and prolonged declines in HIV - has marked a significant upswing in new infections.

Once HIV/Aids begins to go down, there are no guarantees it will stay down. Especially concerning is the growing number of reports of Aids patients who are resistant to the first line of anti-retroviral drugs - work must continue on developing the next line of medications. Each generation must be as vigilant as the last. And South Africa's epidemic shows no sign of abating.

Over the last five years, Aids has garnered extraordinary attention from the world's leaders. That attention is paying dividends. But, twenty-five years into the pandemic, the safest prediction is that Aids will still be with us twenty-five years hence. Our efforts need to be sustained for a generation at least.