US lab sleuths have helped nab a pair of criminals who infected their sexual partners with HIV after tracing the virus from the perpetrators to the victims, according to a study.

In both cases, one in the western state of Washington and one in Texas, the men were found guilty of infecting multiple women with the virus that causes Aids after science confirmed the link.

The findings published in the Proceedings of the National Academy of Sciences describe how scientists were able to narrow down the exact type of HIV that matched both the accused criminals and the women they infected.

The research marks a breakthrough in scientists’ ability to say definitively which person was the source of the infection.

“This is the first case study to establish the direction of transmission,” said Michael Metzker, associate professor in the Baylor College of Medicine Human Genome Sequencing Centre in Texas.

Scientists were “blinded” in the studies, meaning they were unaware which samples came from the accused and which came from the women.

The process was complicated because of the way HIV presents itself in an infected person, said Mr Metzker.

“Within a given person, there is not just one strain but a population of strains because HIV mutates all the time when it makes new virions (viral particles),” remarked Mr Metzker.

“During transmission, however, there is a genetic bottleneck in which only one or two viruses get transmitted to the recipient.”

By narrowing down the single “ancestor” of HIV and comparing two distinct gene regions of the virus across different subjects, scientists were able to reconstruct the history of the virus – in a process known as phylogenetic analysis.