Mini-epidemics of HIV have been recorded among gay men, a study has found.
New data shows distinct clusters or "bursts" of the disease among men in London.
The University of Edinburgh study found many men who became infected with the virus passed it on within a few months, often before they themselves had been diagnosed.
Professor Andrew Leigh Brown of the University of Edinburgh'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."
Researchers now believe specifically targeting bars, nightclubs and the internet could be the best method of curbing the spread of the disease by sexual contact.
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.
"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."