Fifteen-year-old Tadisa was never expected to live. Her mother Grace was HIV-positive and passed the virus on to Tadisa at birth.
In the 1990s, thousands of babies born with HIV in Zimbabwe died in infancy, as today's cheap life-saving drugs were not available in sub-Saharan Africa.
But to the great surprise of doctors, there are thousands of teenagers like Tadisa who have lived with HIV for more than a decade.
When Dr Rashida Ferrand wanted to research these young survivors, she was told none were still alive.
"Five years ago people would shake their heads in disbelief and say: 'Well, no, nobody survives,'" said the HIV expert from the London School of Hygiene and Tropical Medicine.
"Survival beyond five years is considered absolutely exceptional."
No-one knows exactly why people like Tadisa survived childhood with untreated HIV.
But while they have lived longer than expected, their lives have not been straightforward.
Many suffer chronic health complications, including disfigurement, and as a result are socially ostracised.
Dr Ferrand now treats these teenagers in Zimbabwe and is a passionate advocate for them.
"These children are more likely to be poor and to have lost their parents to Aids," she said. "They've been shifted around amongst guardians and missed education, so the odds are stacked against them."
'Untreated HIV infection'
Sub-Saharan Africa accounts for 69% of the world's HIV cases. Today, nearly one in 20 adults live with the condition.
Life-saving antiretroviral therapy (ART) has been available for free since 2004 in the region and has transformed the outlook of those with the virus.
But children like Tadisa who survived unexpectedly have often gone untreated, and the complications of untreated HIV - such as damage to the lungs and heart - are debilitating.
"There's also some brain damage leading to them not being able to perform in school," said Dr Ferrand. "There are some obvious visible signs of longstanding untreated HIV infection, such as stunting and skin disfigurement.
Part of the problem is a lack of awareness: four out of five children with HIV do not know that they have the virus.
Their medical problems are often put down to normal childhood illnesses and they do not get tested for HIV until they are in their teens.
'Culture of silence'
Dr Ferrand described a "culture of silence" and said the "most shocking" aspect of her research was that nearly all the older children and adolescents who have tested positive could have been diagnosed earlier.
Two-thirds had been to a primary health care clinic in the previous six months, a quarter had been in hospital at some point in their lives, and more than half had a parent or sibling with HIV.
Despite this, no-one had thought to test them for HIV.
Once tested, there was no standard way for young people to be told of their status, according to research at Harare's Parirenyatwa Ol Clinic led by Dr Khameer Kidia and including Dr Ferrand.
They asked 31 patients aged between 16 and 20 how they discovered they were HIV-positive.
Although the advice was for their parents or guardians to tell them, the adolescents preferred to hear the news for themselves.
"I wanted to be told at the clinic just so I know that it's really true, that I've been tested, and it's true", said one 17 year-old girl.
Some children had guessed, such as one 17-year-old boy who told the researchers: "My mother was lying to me saying I have a heart problem, I have a hole in my heart. So I decided to say, 'Okay.'
"But I knew. I knew that when I was coming here I was HIV-positive."
And sometimes the news was given in an abrupt manner.
"My grandmother told me at home," said one 18-year-old boy. "I was watching TV. My grandmother came up to me and said: 'Hey, A, do you know that you're HIV-positive?' I said, 'Okay.' She said it twice: 'You're HIV-positive.' I just said, 'Okay.'"
The adolescents learn most about their condition from each other rather than their parents and guardians, the researchers found.
Once diagnosed, they can keep the virus at bay with the free ARTs.
But one reason their guardians did not bring them for diagnosis and treatment was to protect them and their families.
"They fear that if the child is disclosed to, he will go about in the streets or at school telling others and other relatives that don't know that the parents are positive," said a counsellor at Parirenyatwa Ol Clinic. "So they will be stigmatized or discriminated against as a family."
In 2013 a UNICEF report stated that only about a third of children with HIV were receiving ARTs, compared with around two-thirds of adults.
New WHO guidelines say testing should be offered to all adolescents living in areas of the world where HIV is common, particularly sub-Saharan Africa.
Dr Ferrand believes it is an issue of child rights that the young people learn about their HIV status and receive treatment.
Hear Dr Ferrand speaking to Health Check on the BBC World Service.