The HIV virus is becoming increasingly resistant to drug treatments, researchers have warned.
The research looked at different HIV strains
A study of 1,600 patients across Europe found one in 10 patients who have never taken antiretroviral drugs for HIV already had a resistance to at least one of them.
Researchers suggest this can only have happened through HIV-positive patients who are taking the medications infecting others with a drug-resistant strain.
They warn that if these strains of HIV continue to spread, it will limit the drugs doctors can use to treat the virus.
The researchers recommend patients diagnosed with HIV who became infected in Europe should be tested to see if they have a drug-resistant strain.
In the UK, the guidelines on testing are set to be strengthened to recommend anyone newly diagnosed with HIV should be tested to see if they have a drug resistant strain.
The study looked at how resistance was developing in different strains of HIV.
When all the patients were examined, it was found that 10% had drug-resistant HIV.
But when researchers looked more closely, they found rates differed depending on what type of HIV people were infected with.
The research, which looked at HIV patients in 17 European countries, found drug-resistance in 11.3% in patients infected with HIV subtype B.
This type of HIV is much more common in Europe and North America.
Non-B HIV is more widespread in Africa and Asia, though it is spreading to Europe due to migration.
But researchers found resistance levels of just 3.3% in those with non-B HIV.
The international team of scientists behind the study say this difference is due to the fact drugs for the "European" strain have been available for longer, so resistance has had time to develop.
Jack Summerside, health promotion manager at the UK's Terrence Higgins Trust, said: "This research highlights the necessity of ensuring that people with HIV have the necessary knowledge and skills to avoid passing HIV on to others, especially if they have a drug-resistant strain.
"It should not be seen as a rallying cry to name, shame and further stigmatise people with HIV."
Scientists are working to tackle the development of drug-resistant HIV strains.
This month, pharmaceutical company Roche launched its drug Fuzeon in the UK.
The drug is specifically aimed at patients who are not responding to existing treatments.
It works by blocking the virus and stopping it from entering healthy immune cells.
Other drugs attack the virus when it is inside these cells.
The research was presented to the International Aids Conference in Paris.