20 July 2016 Last updated at 08:43 BST
Better education, the availability of drugs that fight the spread of the virus and the prevention of HIV transmission to newborn babies have all played a role.
But it is proving stubborn to get rid of.
As South Africa hosts a major HIV-Aids conference, the BBC's Karen Allen has been to one of the country's HIV hot spots in KwaZulu-Natal.

Prof Wendy Savage told the Mail on Sunday that withholding the information because of fears over sex-selective abortions was "outrageous".
Some hospitals have a policy not to reveal the sex, but Professor Savage said it was a woman's right to know.
She also said it was a "woman's right to decide" when to ask for an abortion.
Speaking after MPs voted to introduce a bill to repeal an 1861 law criminalising abortion this week, she told the Mail on Sunday: "It's her body and her foetus, so she should have that information.
Parents who want to find out the sex of their foetus can usually do so in a mid-term scan at about 20 weeks but some hospitals have a policy of not telling, according to the NHS Choices website.
Some hospitals say they are too short-staffed to establish the sex and there have been concerns that the scans can sometimes be inaccurate.
But experts have also claimed that the policies may be intended to prevent the risk of selective abortions.
"She is the one taking the risks.
"If a woman does not want to have a foetus who is one sex or the other, forcing her is not going to be good for the eventual child, and it's not going to be good for (the mother's) mental health," she added.
Professor Savage, 81, a retired obstetrician and gynaecologist and a member of the British Medical Association's (BMA's) ethics committee, was speaking to the paper in a personal capacity.
Under the 1967 Abortion Act in England, Scotland and Wales, a pregnancy can be aborted before 24 weeks if two doctors approve it.
The law does not apply in Northern Ireland, where abortion legislation is based on the Offences Against the Person Act 1861.
The majority of abortions in England Scotland and Wales take place before 24 weeks, and are allowed in exceptional circumstances after that time.
More than 90% take place at 13 weeks or earlier, according to the BMA.
Prof Savage added: "The foetus is a potential human life at that stage (in the womb); it is not an actual human life... I think you've got to concentrate on the (rights of the) woman."
Prof Savage said in her long career she had "only a couple of cases" of being asked to approve an abortion past 24 weeks.
"It's not something women tend to do," she said.
"This is another myth propagated by the anti-abortion lobby, like women wanting sex selection."
She also advocated making abortion pills, for early termination, available over the internet, without it being mandatory to consult an expert.
MPs last week voted in favour of progressing the Reproductive Health (Access to Terminations) Bill.
It would remove criminal sanctions over abortion for women and doctors in England and Wales.
A spokesman for the Society for the Protection of Unborn Children said although the comments came in a personal capacity they were in the context of the "BMA's exploration of further abortion liberalisation".
And, he said, in light of a recent Royal College of Obstetricians and Gynaecologists event presenting widespread abortion "as a matter for celebration".
"Such views have nothing to do with medicine, the care of mothers or concern for the sanctity of life," he added.
A BMA spokesperson said it supported the current law on abortion, adding: "Though we recognise the diversity of opinion amongst membership, we advise members to act within the boundaries of the law and their own conscience.
"Given the range of views on this subject, patients must be entitled to impartial and objective medical advice and treatment."
Remarkable progress has been made in reducing the rate of growth in new HIV infections - the virus that causes Aids.
[NEXT_CONCEPT]
Women should be told the sex of the foetus when they are scanned during pregnancy, a senior, retired obstetrician has said.