There are few examples from the early days of the epidemic of countries that initiated successful prevention measures. One often quoted is Uganda, the first country in Africa to be seriously affected.

In 1986 President Yoweri Museveni responded to the emerging HIV crisis in Uganda by embarking on a nationwide tour to tell people that avoiding AIDS was a patriotic duty, and that they should abstain from sex before marriage and then go on to remain faithful to their partners and to use condoms. Uganda's Health Minister announced to the World Health Assembly that there was HIV in Uganda, and the first AIDS control program in Uganda was established. It focused on providing safe blood products, and educating people about risks.

Museveni’s campaign gave rise to two famous slogans which have come to be used throughout Africa: ‘ABC’ (Abstinence, Be faithful, use a Condom) and “Zero grazing”, meaning not to have sex with casual partners.

HIV prevalence fell in Uganda from 21% in pregnant women and 15% in the general population in 1990/1 to around 4% in 2004, though there is much argument as to whether this is due to the HIV prevention programme or to natural attrition through people dying of AIDS – see epidemiology for more details – and the national figures have been disputed by independent activist bodies like the National Guidance and Empowerment Network.

In Thailand the “100 per cent condom campaign” in 1990-92 is often seen as one of the most successful HIV prevention programmes of all time. It slashed HIV incidence among young men from 2.5 per cent a year to 0.5 per cent, reduced prevalence among army recruits from 10 per cent to 2.5 per cent, and it is estimated that HIV prevalence in Thailand today is still – a decade after the campaign ended - 50 per cent lower than it would have been if it had not happened.

Its success was partly due to good timing and an accurate perception that a widespread culture of commercial sex was responsible for the rapid growth of HIV at the time. It was also partly due to it being an easily enforceable target. The campaign put pressure on brothel owners to enforce 100 per cent condom use in their establishments and ensured that ones not conforming to this rule were closed by the police.

Other countries like Brazil and Senegal, who have managed to contain HIV prevalence to less than 1%, are perhaps less celebrated because they never allowed HIV to get out of hand and become a generalized epidemic. An example of how a country has to take radical steps to contain HIV comes from Senegal, where in the 1990s a number of HIV cases were traced to transmission within all-male Islamic seminaries. Though obviously this was due to male-male sex, the subject of homosexuality was never specifically addressed, but HIV education posters and classes were introduced into a setting that would in some other Islamic countries be completely inadmissible.

It is perhaps more instructive to look at countries that failed to institute timely prevention programmes to see what can happen. One example is South Africa, where an epidemic mainly concentrated among the country’s gay population turned into one of the worst generalized epidemics in the world. This was partly due to the anti-apartheid struggle and the institution of democracy reaching their peak and distracting attention at the time HIV was taking hold, and an ambiguous and lukewarm government attitude towards prevention and especially treatment since.

A current example may be Russia, were HIV prevalence increased sevenfold between 2002 and 2004. Another may be the USA, where the failure to take HIV messages to new populations at-risk has led to a hugely disproportionate HIV prevalence among the US black population.

See Epidemiology and prevention for more data on developing countries.