World Aids Day: I met Enoch. Three years ago he couldn't stand up. Now his life is transformed. The cost? 23p a day

I choose not to listen to the cynics, whose philosophical opposition to aid bewilders me

What value can you put on a human life: £1m, £1bn, priceless? How about 23p a day? That's what it costs to provide a Malawian with HIV with life-saving anti-retroviral drugs. And the really depressing thing is that, for two-thirds of Malawians with HIV, that price is too high. No one is willing to stump up the cash needed to keep them alive.

Think what you can buy for 23p: a Mars bar costs 60p, a pint of milk is 50p. And then realise that 650,000 Malawians with HIV cannot get the medicines they need. If I sound angry about this it's because I am.

I recently returned from Malawi. This is a country poor even by African standards, and one which is ravaged by HIV. One million people – 1 in 12 of the population are living with the virus.

I saw first hand the difference a tiny amount of aid money can make to the lives of people who would otherwise be living with a death sentence.

Enoch, a farmer in his 60s who was diagnosed with HIV 10 years ago, told me: "If you'd seen me three years ago you wouldn't think I was the same person. I was very, sick, I couldn't stand up. I'm alive today because of the medication I receive."

Some people worry that our aid budget is somehow being wasted, that it doesn't reach the people who need it or that aid is somehow counterproductive. Others express concern that we won't be able to spend the increases in aid that our Government has promised for 2013 – the year we will finally meet a 40 year-old promise to the world's poorest to spend 0.7p of every £1 of the UK's national income on aid.

I choose not to listen to the armchair cynics, whose philosophical opposition to aid bewilders me. I choose to listen to people like Enoch, and Mara Banda – who also told me, quietly and simply, that she would be dead today if it were not for medicines paid for by our aid. We can and should be helping more people like them. World leaders had agreed that by 2010, the world would pay for medicines for everyone with HIV. Yet almost 7 million people – and a shocking three-quarters of children – living with HIV cannot get the treatment they need because of a lack of funding.

Failure of governments and other donors to fund adequately the Global Fund to Fight Aids, Tuberculosis and Malaria has left that body with a £1bn hole in its budget.

We are in danger of undermining what is a real success story. The Global Fund has been a modern-day miracle, using aid money to get nearly 4 million on to life-saving medicines. Five years ago virtually no one could access these medicines and HIV/Aids was a definite death sentence. But the Global Fund can barely afford to keep funding existing treatments.

In 2011, there were still 1.7 million Aids-related deaths – that's equivalent to the combined populations of Birmingham and Leeds. In some ways, Enoch is lucky he did not fall ill this year, as the Global Fund has no money for new patients.

That progress may be at risk not only because individuals will not get the treatment they need, but also because anti-retrovirals decrease the chances that people with HIV will pass on the virus to others. Over time, today's funding squeeze by donors could end up being very costly.

You often hear the argument that because other countries are failing to deliver on their promises, this should somehow let us off the hook. In fact, in this climate, David Cameron deserves ever more credit for sticking to his guns in the face of an onslaught by critics.

And it is ever more important that he does so. Not only because at a time of dwindling aid budgets, the UK's becomes proportionately more important, but also because as long as the UK keeps its promises, it makes it harder for leaders in other developed countries – like France, Germany or Canada – to explain why they can't deliver on theirs.

But forget the politics; this is a question of basic humanity. When you hear someone explain to you how they've been saved from death by a simple act of charity, then the only healthy human reaction is to want to extend that same charity to others facing the same plight.

Especially when it costs less than a pint of milk.