Study leader Andrew Leigh Brown and colleagues at the University of Edinburgh and London's Chelsea and Westminster Hospital said people frequently passed the HIV virus to others within months after becoming infected themselves.

The study, published in PLoS Medicine, found the growth of HIV among men who have sex with men in London was not a slow and steady process, but episodic. With multiple clusters of transmission occurring within a few years during the late 1990s, the number of HIV infections in this population doubled.

Genetic data on HIV is routinely obtained to best determine HIV medications, therefore Leigh Brown and colleagues were able to compare the sequences of HIV genes from more than 2,000 patients, mainly who attended a large London HIV clinic from 1997 to 2003.

Of the sequences analyzed, 402 closely matched at least one other viral sequence. Further analysis showed the patients whose viruses showed matches with others formed six clusters of 10 or more, as well as many smaller clusters.