An Australian teenager stabbed his mother's boyfriend to death after she received prank phone calls alleging that the man may have infected her with HIV, The Sydney Morning Herald reports.

Jason Podesta, who was 19 at the time, admitted to the Australian Supreme Court that he killed Steven Bannister, 24, on November 22, 2006. However, he pleaded not guilty to murder on the grounds of mental illness.

Earlier that evening, Podesta's mother, Cathy, had received a number of prank phone calls from an unknown man suggesting that Bannister was HIV positive.

According to Christopher Maxwell, Queen's Counsel for the Crown, Podesta told police that his mother was “extremely upset about the calls and that she may have contracted the disease.”

Maxwell told the Supreme Court jury that Bannister died as a result of three wounds to the front of his chest and one on his back. There were 20 stab wounds on his body in total.