An accused child sex offender has been granted a new trial over claims he was convicted on prejudicial evidence that he and the victim both tested positive for herpes.
The High Court quashed Malcolm Laurence Orreal's convictions and ordered a new trial in a decision published on Thursday.
In a jury trial in the Queensland District Court, Orreal was convicted of three counts of indecent dealing with a child under 16 years and two counts of digital rape.
His alleged victim was 12 years old when she claimed Orreal sexually assaulted her. She lodged a police complaint the next day.
Medical evidence found the assault was consistent with blunt force trauma and a traumatic break of her hymen.
The prosecution presented evidence indicating that both Orreal and the complainant had tested positive for the Herpes simplex virus type 1 (HSV-1) during the trial.
Orreal tested positive through a blood sample, while the complainant tested positive through a vaginal swab.
A paediatrician gave evidence that HSV-1 causes cold sores but can be found as a genital infection with the virus remaining in the infected person's body.
The virus can spread by oral-to-genital spread and genital-to-genital spread from someone who is "shedding" the virus.
The trial judge directed the jury to "take that evidence into account along with all of the other evidence" and Orreal's counsel also failed to object to the medical evidence.
Orreal successfully challenged the conviction, arguing that admission of HSV-1 evidence was "highly prejudicial".
"All that could be said about the appellant was that at some point in the past he had been infected with HSV-1," the High Court found.
"It was not possible to say whether he was shedding the virus when the alleged offences occurred.
"Likewise, it was not possible to say when the complainant acquired the virus or from whom she acquired it."
The court found the HSV-1 evidence had no probative value and was inadmissible.
"The absence of any clear direction from the trial judge to the jury to disregard the impugned evidence left the jury free to 'make of that' what they would.
"In effect, the jury was invited to employ the impugned evidence as they saw fit."
Orreal is now expected to face a retrial at a later date.