"I've been really lucky," says Brent Stirton of his double win at this year's World Press Photo. "This year there were many big news events so I'm glad I was able to draw attention to other stories."

Stirton's stories happened far away from the tsunami and revolutions that dominated the 2011 news pages - he picked up first prize in the Contemporary Issues Singles category for a shot of a Ukrainian sex worker and drug addict called Maria, amd in the Nature Stories with a series of images on rhino horn hunting in South Africa. The portrait of Maria is actually part of a larger series on HIV in the Ukraine that was singled out by the World Press Photo judge, but Stirton says he didn't mind.

"I just feel like 'Thanks for paying attention to this issue'," he says. "I think a lot of photographers are very self-indulgent over their stories - I don't see a lot of good stories. The most important question is, 'Does it tell me about the issue?' I want each image to have a very specific function and in that, I am a journalist before I am a photographer. A single image can still tell you about the story, and it can be a taster for a longer essay."

Stirton has been working with an HIV charity Global Business Coalition Against AIDS for nearly 10 years, and has been photographing in the Ukraine since 2005. He says Maria's story sums up the experience for many sex workers in the region, forced into the role by addiction or poverty, and adds that she told him to take the photograph.

"She was very matter of fact," he says. "She said 'Take the picture, it's a warning - people should see what happened to me and take it as a warning'. I was very impressed that someone so exhausted could still have that defiance and dignity. She's in the grip of a serious addiction in a society that doesn't have the means to help her, so she's very fatalistic. That really struck me when I took the photograph."

Stirton won the Nature Stories category with an essay on a completely different subject - White Rhino horn hunting in his native South Africa. This story was shot for National Geographic and also required considerable co-operation, from both the people trying to save the animals from extinction and those involved in buying White Rhino horn. "It's believed to have medicinal properties and is now consumed in such tremendous quantities it is worth the same as gold," he says.

"WWF helped me and I worked with a vet who introduced me to people who could help me photograph a rhino that had been de-horned. I also went to Vietnam to show how the product is value - in total I worked on the story for eight weeks. It was published by National Geographic with lots of statistics and maps and a very well-written article, but one of the pictures was also released last year and it generated a lot of interest.

"To me, the internet is the most important medium because it allows so many people to access the images, but I'm also glad to be exhibiting my work with World Press Photo [which will take a WPP 2012 exhibition around the world]. It's always a pleasure to see my work printed well and shown at large size."