By Joanna Buchan BBC News

From an obscure reference in a specialist medical journal to a world-wide health crisis, Aids - Acquired Immune Deficiency Syndrome - has transformed the way we live.

In the 21 years since it first appeared, official figures say that 24 million people have died around the world.

When the Aids epidemic first began, no-one was really sure about how HIV could and could not be caught

Another 40 million are currently infected with HIV, the virus commonly believed to cause Aids.

Billions of dollars have been spent, and yet there is no vaccine, no cure, and apparently, no end in sight.

This year's UN report into the disease is the most depressing yet.

Global crisis

For nearly 20 years I've been listening to people talk about Aids - doctors, patients, scientists, politicians.

They have been scared, disdainful and angry.

In that time, Aids has gone from being something whispered about in small voices, to being recognised as the greatest global health challenge we have ever faced.

In the developed world Aids was seen as a gay disease

Yet, in the 22nd year of the pandemic, we are still very much at risk.

It is generally accepted that in the developed world, Aids was first noticed in San Francisco and New York in 1981 - it was seen exclusively in gay men, and was known as Grid - Gay Related Immune Deficiency.

By 1984, when I began to report on Aids, Edinburgh, the city where I worked, was the Aids capital of Europe. Drug addicts who injected and shared needles were the first infected there.

Public apathy

It took a while for the British Government to decide what to do about this new disease, and when the public health campaign did finally get under way in Britain the people I spoke to were generally unimpressed.

In the developed world we have forgotten that there was ever a reason to be afraid

Why waste money on something like that? They believed they had nothing to worry about - they saw Aids as something that happened only to gay men, drug addicts - others.

When the Aids epidemic first began, no one was really sure about how HIV could and could not be caught.

The fear - and the stigma - caused panic in some places. In one case in America, a 13-year-old haemophiliac, Ryan White, was banned from several schools after his HIV positive status became public.

Media hysteria

Matters were not helped by the hysterical coverage of deaths like film star Rock Hudson's, in 1985.

Ryan White was banned from schools

Looking back at those years, it was a time of great fear and of equally great denial. The doctors and scientists I talked to were genuinely scared of what was coming - the general public was mostly oblivious.

But of course there were really two epidemics developing.

At the same time as the gay community in the developed world was becoming organised and politicised as never before, taking on the community health issues for themselves, a far more widespread catastrophe was engulfing Africa.

Gay resistance

On a visit to New York in the early 1990s, I saw red ribbons worn as a symbol of resistance and pride for the first time.

There was immense ignorance even among doctors

I met a bunch of vocal, and determined gay men, who were raising funds for Aids work in the community which amounted to more than the health budgets of some small nations.

At the same time, hundreds of thousands in Africa who had "slim disease", first identified as Aids in Uganda, Rwanda, and Zaire, could not afford the cost of a headache remedy.

Slim was a silent killer then, whole villages vanishing on a continent driven by famine and war - who was to say what had killed them?

Reversing the trend

There have been success stories of course, and Uganda is the most notable. There the political will existed early, and at the very highest levels.

Africa is bearing the brunt of the pandemic

The result of a sustained campaign of education and prevention has been that infection rates dropped there in the 1990s, the opposite of what was happening everywhere else, and they are still dropping.

For South Africa the opposite has been true, and much of the fight against HIV and Aids there has not been about the cost of drugs, but about President Thabo Mbeke's insistence that HIV does not lead to Aids.

Sub-Saharan Africa is home to two-thirds of all HIV positive cases. South Africa has the largest number of people living with HIV and Aids in the world.

Subsiding fear

What has changed in 21 years?

The cost of drugs is a massive problem

I remember sitting with a newly diagnosed patient in a leading London hospital in 1986, and watching doctors and nurses put on space suits before they would come in to her closed room and take her temperature.

I remember when reading the United Nation's report into the Aids pandemic would guarantee me a seat and elbow-room on the most crowded commuter train.

Now HIV and Aids is part of daily life and, in the developed world, we have forgotten that there was ever a reason to be afraid.

Inequality prevails

But for the majority very little has improved. Equality of treatment remains a dream.

Despite the cuts in some drug prices, the cost of treatment now starts at around $350 a year. Still too much for the poorest - and they still bear the brunt of this disease.

In sub-Saharan Africa last year, 30,000 people were able to take the latest drugs, but two million people died of Aids.

This week's international conference on Aids is hoping to find energy, inspiration - and money - to send delegates out refreshed, to fight on.

But the government leaders and heads of state who will be at the conference are from the countries desperate for help. The leaders of our richest nations will be staying at home.