Bleak findings will worry many who thought anti-retrovirals were the key to a better life

Most people carrying HIV in the United States take a cocktail of three drugs to keep the virus under control.

When the virus within an individual evolves resistance to all three, the result is a complete breakdown of the immune system - the syndrome of Aids.

New drugs needed

The new figures from the United States are not in themselves surprising.

Research in other parts of the world has produced similar findings.

But this is a much bigger study than those performed previously, and big studies produce reliable data.

Experts say the findings have two major implications for treatment and research.

Firstly, they demonstrate the need for new classes of drugs to tackle the virus in new ways - and, as other researchers have been telling the same medical conference, a new type called fusion inhibitors could be available within a year.

Secondly, the study reinforces the importance of safe sex, and safe use of needles by intravenous drug users.

The more people who carry drug-resistant strains of HIV, the more likely it is that anyone being newly infected will pick up a drug-resistant strain, reducing their chances of successful treatment.