Campaigners have fought to make the drugs available
HIV, the virus that causes Aids, is fast becoming resistant to the drugs used to treat it, according to research presented at a major medical conference in Chicago on Tuesday.
Scientists have discovered that more than three-quarters of HIV-positive people in the United States carry versions of the virus that are resistant to commonly used drugs.
The researchers examined viruses taken from about 1,500 HIV-positive people across the US.
They found that 78% of them carried HIV which has become resistant to at least one anti-retroviral drug, while more than half had a virus that was resistant to more than one.
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 condition known as 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.
However, 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.
First, 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.