Health officials in New York say they have found a new strain of highly drug-resistant HIV in a city resident.
New York health officials are looking for other cases of the strain
The resident, a man in his mid-40s, is thought to have developed Aids much faster than usual after infection.
The strain - known as 3-DCR HIV - has not been detected anywhere else in the world and is "difficult or impossible to treat", according to health experts.
The discovery prompted renewed warnings from city health officials for people to practise safe sex.
New York health workers are now hunting other possible cases of the strain.
Experts say drug resistance is increasingly common among people with HIV, but not when accompanied by such a quick progression to Aids.
Officials said he frequently used the drug crystal methamphetamine, a stimulant taken to reduce sexual inhibitions.
The man - who has not been named - is thought have been infected quite recently, and developed Aids between two and 20 months after infection. Progression to Aids usually takes about 10 years.
The strain was found to be resistant to three out of the four commonest anti-retroviral drugs.
Ronald Valdiserri, director of the HIV/Aids programme at the US Centers for Disease Control, no similar case had been found anywhere in the world.
"What's unique about this is the combination of multiple drug resistance and a rapid course," he told the New York Times newspaper. "To folks in the public health community, that is a particularly dangerous combination."
However, some Aids specialists outside New York have cast doubt on the alarm, saying that the man's immune system may already have been compromised, the newspaper said.