The virus HIV is passed through direct contact with body fluids - most commonly during sex, blood transfusions and by injecting drugs.
Despite two decades of research there is still no known cure for the disease, and more than 20 million people have already died because of it.
UN Secretary-General Kofi Annan said: "There is no more important issue for us to consider than Aids."
One of the main messages of World Aids Day will be to prevent the spread of the disease by practising safe sex through using condoms.
The virus HIV is passed through direct contact with body fluids - most commonly during sex, blood transfusions and by injecting drugs.
It is not regarded as highly contagious but, since its discovery in the early 1980s, the number of people living with the disease has risen to more than 36 million and is expected to rise by another six million over the next year.
China 'epidemic' warning
Officials in China on Friday handed out condoms and newspapers carried articles about Aids victims.
UN experts have warned that although the government understands the size of the problem, China could be facing 10 million cases of HIV/Aids within a decade.
"Unless there is really substantial action in the next three or four years, the real danger is that we will have 10 million cases in the year 2010 or worse."
In Vietnam, one of Asia's worst-hit countries alongside China, Cambodia and Burma, UN workers and officials launched a campaign intended to increase awareness of HIV and Aids.
Pink buses decorated with condoms and red ribbons took to the streets of Hanoi and Ho Chi Minh City before setting off on a tour of Vietnam's major cities to spread the safe-sex message.
In Australia, children of Aids patients in the city of Melbourne planted a new rose hybrid - the Hope Rose - in a ceremony intended to inspire those who have the disease not to give up.
Former South African President Nelson Mandela urged people in his country, which has one of the world's highest rates of infection, to use condoms and be more open about Aids.
"Give a child love, laughter and peace, not Aids," he said.
Mr Mandela said Aids was threatening the very fabric of South African society. The issue is particularly sensitive in South Africa as his successor, Thabo Mbeki, has questioned the link between HIV and Aids.
Spiralling infection rates
More than three-quarters of the world's Aids related deaths have been recorded in sub-Saharan Africa.
But the United Nations organisation, UNAIDS, says new regions are seeing infection rates spiralling out of control.
These include Russia, where there are over a million injecting drug users, and Eastern Europe where drug use and prostitution are on the rise.
In the West, anti-viral drugs that can slow the progress of the disease almost to a halt are routinely used but they are too expensive for developing countries.
And in Western countries health workers are fighting a constant battle against complacency brought about by the success of anti-viral treatments.
There are a number of potential vaccines to prevent infection on trial around the world, but researchers warn that a workable vaccine is at least a decade away.