HIV/AIDS has claimed around 35 million lives worldwide. But now, as Dr Chris van Tulleken reveals, cutting-edge science can keep the virus at bay or even prevent infection altogether. As a new preventative treatment called PrEP is rolled out on the NHS in Scotland, and new trials are announced in England and Wales, HIV is under control, in Britain at least, but only when it can be detected and the treatment with antiretroviral drugs (ARVs) can begin. Chris meets a woman whose husband died without ever knowing he had the disease, by which time he had infected her too. But Chris also finds out how 'viral loads' can now be reduced to allow patients to lead healthy lives - and even prevent them infecting anyone else. Chris meets HRH Prince Harry for an interview at the Mildmay Hospital, an HIV hospital made famous by Diana, Princess of Wales, where Chris also meets a patient whose undetected HIV led to serious brain damage. With seven out of ten of people infected worldwide living in sub-Saharan Africa, Dr van Tulleken visits a clinic in KwaZulu-Natal, South Africa, where tens of thousands are still dying. He meets an infected schoolgirl and realises that local attitudes to testing are still leading to unnecessary deaths. But Chris also meets clinicians taking mass testing out to the villages and meets a man whose life was saved as a result. Back in the UK, talk of a cure may be premature, but Chris finds out more about the controversy around the rollout of PrEP which, when taken daily, can prevent someone becoming infected in the first place.