HIV is spreading among gay men in mini-epidemics, a new study has found.
Data from more than 2,000 infected men in London showed distinct clusters or "bursts" of the disease.
Researchers now believe targeted local campaigns in bars, nightclubs and via the internet could be the most effective way of curbing the spread of the disease by sexual contact.
The University of Edinburgh study was carried out with Chelsea and Westminster Hospital in London, using data collected between 1997 and 2003.
It found many men who became infected with the virus passed it on within a few months, often before they themselves had been diagnosed as HIV-positive.
Professor Andrew Leigh Brown of the University of Edinburgh's school of biological sciences, who led the study, said: "What we have discovered is that some of the spread occurred in bursts, with groups of people becoming infected within a short period of time."
Prof Leigh Brown said such a pattern had been seen occasionally among HIV-infected drug users but had not been identified in sexual transmission until now.
He added: "The tightness of clusters that we have found is frightening. The results raise concerns that a drug-resistant version of the virus could spread quickly, causing a mini-epidemic which is hard to treat."
Prof Leigh Brown said the study findings indicated that the safe sex message was "not getting through".
The research is published in the Public Library of Science (PLoS) Medicine journal.
