Aids is now the leading cause of death in Africa, overtaking malaria as the continent's main killer disease, the United Nations has said.

It said the epidemic was responsible for one in five of all deaths in Africa last year.

Worldwide, the UN said Aids is now a bigger killer than any other infectious disease - and the fourth overall cause of death, after heart disease, strokes and respiratory infections, which often affect people in old age.

Nowhere in the world has the impact of Aids been more devastating than in Africa, where the UN estimates that more than 11 million people have now died of the disease.

'Formidable threat'

Peter Piot, executive director of UNAids, the body responsible for co-ordinating the fight against Aids, says it is the most formidable disease to confront modern medicine, with the potential to undermine the massive improvements made this century in global health and well-being.

He said HIV was catastrophic for two reasons: "Aids targets young adults and the number of deaths are accelerating quickly.

"Even if we stopped HIV today, because of the millions of people now living with the infection, the burden of Aids will continue to be severely felt.

"This is only the tip of the iceberg," he said.

Millions died

The UN estimates that two million Africans died of Aids last year. This is well over 80% of the world-wide death toll.

Malaria, which used to be Africa's main killer, accounted for about a million deaths.

Part of the explanation behind the jump in the UN's Aids figures is statitistical - many deaths previously attributed to tuberculosis are now known to have been actually caused by Aids.

Life expectancies in many countries are now plummeting as a result of, and national health systems have been overwhelmed.

Earlier this year, the Burundian government had to send Aids patients home from hospital because, it said, there were almost no beds for anyone else.

Aids cases continue to rise

The report - prepared in association with the World Health Organisation - said that while only small fluctuations in impact have been seen over the years with other causes of death, the Aids curve was rising sharply.

"These new findings challenge the world to make better use of the tools we have to reduce the impact of Aids, including prevention and care, and to speed up the search for an Aids vaccine," Mr Piot added.

Another finding of the report is that depression and other disorders which affect the brain have a major impact on a person's physical health.

The WHO measured how disability and other conditions affected ill health.

They found neuropsychiatric conditions, including depression, dementia and alcohol and drug abuse, accounted for 23% of the disease burden in Europe and the USA.

The report said: "Neuropsychiatric conditions have been ignored for a long time as they are absent from cause of death lists.

"However, when disease burden measurement includes time lived with a disability, several of the neuropsychiatric disorders become leading causes of disease burden worldwide."

The WHO estimates that major depression shortens healthy active life by four years.