By Darryl Chamberlain
Entertainment reporter, BBC News
It was a chance meeting in South Africa that gave Annie Lennox the inspiration for the stand-out track on her new album, Songs of Mass Destruction.
Annie Lennox hopes Sing will raise awareness of Aids in South Africa
Attending a function for former president Nelson Mandela's 46664 HIV/Aids campaign, she saw a man in a black T-shirt with "I am HIV positive" written in big white capital letters.
"It made me curious - that looks pretty hardcore," the singer recalls.
"I asked the person next to me, and he said that was Zackie Achmat - one of their leading activists. He goes to court, he challenges the presidency and the health minister, he went on a hunger strike. And I thought, that's my man."
The end result was Sing, which features a chorus of 23 of music's best-known female singers - and a little bit extra from Madonna.
The activist was from the Treatment Action Campaign (TAC) which, alongside Mr Mandela's 4664 campaign, is fighting for healthcare and education for those affected by HIV and Aids.
Some 5.5 million South Africans are HIV positive - more than in any other country.
But the country's government has been criticised for promoting a diet-based approach to combating the effects of HIV, instead of fully endorsing the use of anti-retroviral drugs.
While Lennox, 52, had worked as an ambassador for 46664 for some time, meeting Mr Achmat gave her the opportunity to get more deeply involved.
"We need people like him, he fights the fight," she says. "He refused to take his anti-retroviral medication unless it was made affordable and available to everyone - a hugely courageous thing to do.
"Before then, I'd been frustrated because I wanted to be more hands-on. I just feel that TAC are doing it where it needs to happen. It really needed to be given support, and I thought that perhaps I might be well-placed to do it."
Lennox was given a CD of music by a group of activists called The Generics - because TAC has helped import generic drugs from Brazil into South Africa in defiance of patent laws.
And the pop star was inspired to incorporate one of their songs into her own.
The project ended up involving a host of female stars including Joss Stone, Shakira, Celine Dion, Gladys Knight, Pink, Sugababes, Anastacia and Bonnie Raitt.
"I'd recorded a track, and was trying to think about how to develop it," she recalls.
"I just needed to ask some other women who were renowned and successful artists. So I got a generic letter, sent it out to them, and waited to see what would happen.
"Some weren't available or were too busy, but they were all so sweet and said they'd love to do it. At 23, I thought, that's enough, because there's everybody and their auntie on there!"
Fitting everybody on proved a challenge, so Lennox asked them to perform the chorus, which emerged "like a big choir".
Lennox was among the stars who performed at 2005's Live 8 shows
She got a little more from Madonna, though.
"Lo and behold, Madonna's track came back and she'd sung the second verse, which was a huge bonus. I was really touched - for Madonna is very rigorous in what she gets involved in and for her to do that for me, I was thrilled to bits."
Songs of Mass Destruction, a mixture of the epic and the haunting, is only Lennox's fourth solo album in a little over 15 years.
It is produced by Glen Ballard - who worked on Alanis Morissette's Jagged Little Pill - who was introduced to Lennox through her old Eurythmics bandmate Dave Stewart.
"I met Glen, and I thought he's such an interesting person, very much someone I could relate to creatively," she says.
They worked together for six weeks in Los Angeles, "which was no bad thing".
"I love it. He was very encouraging, and always made me feel that what we were doing was thrilling," Lennox says.
Sing is intended to be an ongoing project. Lennox says the memory of a choir made up of children orphaned by HIV and Aids, making "music for music's sake", will keep her focus on South Africa.
"It's so moving," she says. "You see these little children, they're beautiful and clean and dressed nicely and singing.
"When you go to Africa, and you see children, they're usually barefoot, dirty and in rags, and they'd love to go to school.
"I'd love to do something for them. There's plenty of possibilities."
Songs of Mass Destruction is out now, while Sing is available to download.