HIV is spreading among gay men in mini-epidemics, a study has found.
HIV poster campaigns have been used to target gay men
Data collected from more than 2,000 infected men showed there were distinct clusters or "bursts" of the disease.
Researchers now believe targeted local campaigns in bars and nightclubs could be the most effective way of curbing the spread of HIV by sexual contact.
The Edinburgh University 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.
Prof Andrew Leigh Brown, of Edinburgh University's school of biological sciences, said: "By studying changes in the virus over time, we have been able to pinpoint its progress in stages through the groups of men affected, which until now has not been done effectively.
"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.
"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."
The findings indicate that the safe sex message was not having a significant impact according to researchers.
The study is suggesting that specific bars and nightclubs could be targeted to get the message across.
"It is important that information on the virus is available to gay men in the local areas where they are known to meet, to try to arrest the spread of HIV and Aids," he added.