The young boyband singers have teamed up with Britain's Comic Relief charity to record the organisation's official Red Nose Day single, a cover of Blondie's One Way Or Another.
They headed to Africa last month for charity work to promote Comic Relief and footage of the trip was shown on Saturday night- and two of the stars were left overwhelmed by the scenes they witnessed.
Styles met a critically-ill toddler, and through tears he declared: "That little boy is too weak to hold his head up, or even cry. It's so sad. He's lying there seriously sick, when a mosquito net could have protected him.
"Our parents worry about what we'll be when we grow up but here the parents worry about their babies getting pneumonia and diarrhoea, which can both be prevented with £5 vaccines."
Malik met a young boy who was being brought up by his grandmother following the death of his mother from pneumonia, and the star was so distraught by the meeting he left the hospital to regain his composure.
He explains: "Looking at that little baby completely helpless with no parents to take care of him, I couldn't help but think about my little sisters and what would have happened to them if they were in the same position.
"We all waste money but the most important thing we can all do is give £5 to protect children from these illnesses."
Comic Relief's Red Nose Day takes place on 15 March, while One Direction's charity single goes on sale on Sunday, and bandmember Louis Tomlinson wants the group's fans to support the fundraiser.
He adds: "We know the proceeds of our Red Nose Day single will help protect babies but everyone can do their bit, too. For less than a fiver, the price of a coffee and piece of cake, you can save a little life - and that's the best feeling in the world."