One year after the attack, a memorial plaque was unveiled

Anyone who ever met Sergio Vieira de Mello will not forget him.

He was a man of immense personal charm, but also hard-headed and pragmatic.

It is well known he had serious doubts about his posting to Iraq - he probably realised sooner than anyone else what an impossible job it would prove to be.

But Kofi Annan insisted, Sergio de Mello went, and by all accounts, he did his absolute best.

When news of the bombing reached Geneva a year ago, there was shock - this city is home to the UN's major aid agencies.

Their staff waited impatiently in Jordan during the US-led invasion, frustrated that they could not enter Iraq to provide aid.

As soon as President Bush declared hostilities over, they went in. And were bombed before they could get a single significant aid project off the ground.

One day after the bombing, Dr David Nabarro arrived back in Geneva - he is the World Health Organization's director for Health Action in Crises.

He came to the UN's famous Room Three - an elegant wood-panelled hall with tall windows and a mural of swans on a lake.

This is the press room, where we correspondents in sleepy Geneva come to hear the aid agencies tell us about the world's woes.

In a voice betraying no emotion, David Nabarro described the Baghdad bombing - he had been in the UN office for a meeting.

The death of Sergio Vieira de Mello and other aid workers brought new dangers into the spotlight

After it, he explained, "I was going to visit my old friend Nadia Younis" - she was working with Sergio de Mello. "There was a sudden loud bang," he continued, "the lights went out, there was dust everywhere, and people screaming."

When he got out of the building, David Nabarro found the grounds already full of the dead, and the dying.

When he looked up to the heart of the bomb blast, he realised "Nadia was in there, Sergio was in there."

David Nabarro talked non-stop for 10 minutes - a graphic, horrific account of that afternoon in Baghdad last year. The Geneva journalists, who often have an irritating habit of asking long and irrelevant questions, were, for once, completely silent.

Later, Sergio de Mello's press officer also came to Room Three, to tell us about funeral arrangements.

He sat down in front of us, he opened his mouth to speak, closed it again, and wept.

Grief primarily for his friend and colleague, but also perhaps for that loss of innocence - the realisation that trying to bring help and humanity to the world's trouble spots doesn't always make you friends.

Dangerous times

One year on from the bombing, David Nabarro still works for the UN.

When I stop by his office he's just back from Darfur in Sudan, and he's shouting down the phone to a colleague there: "What do you mean you can't? There's no such thing."

But he admits: "My life changed on 19 August last year - I've had to realise that I'm at risk, that my colleagues are at risk, and that if someone is determined to kill us, there's not a lot we can do about it."

Aid work has always had its dangers - there are landmines, kidnappings, drunken trigger-happy warlords.

But in the last year, there is a new dimension - the deliberate targeting of aid workers - in Iraq, and in Afghanistan, someone wants them out of the way.

And not just UN workers - last October, the Red Cross, which above all other organisations insists on its independence and impartiality, had its Baghdad office bombed too. "Why? Why do they kill us?", says one Red Cross worker. "Don't they realise we are just trying to help their people?"

The answer may be much more complicated.

Pulling out

Five doctors with Medecins Sans Frontieres were murdered in Afghanistan this June.

What seem to be emerging are no-go areas for humanitarian work

They had been running a mother and baby clinic. Now, MSF will leave Afghanistan after 25 years. Their head of mission, Volker Lamko, is bitter about what he calls the blurring of the lines between independent humanitarian work and military objectives.

As an example, he points out that in Afghanistan, coalition forces distributed a leaflet telling the population they had to provide information about the Taleban and al-Qaeda if they wanted aid deliveries to continue.

Other observers believe that, in the world post-11 September 2001, aid agencies, despite their best efforts to stay neutral, have been compromised - US Secretary of State Colin Powell once referred to aid workers as "part of our combat team".

Not a helpful comment for aid workers in countries where the US presence is resented by large sections of the population.

What seem to be emerging are no-go areas for humanitarian work.

"It distresses me greatly," David Nabarro told me, "because it means we can't help people in dire need."

But the fact remains that neither the UN nor the Red Cross have returned to Baghdad, and aid agencies in Kabul are closing down their offices.

While the attacks on aid workers continue, that's not likely to change - in the meantime, ordinary people desperately in need of aid will have to wait.

A fact which would have filled Sergio Vieira de Mello with deep sadness.