Dr. Leonard Thorne appears in The Pathos in the Pathogens. He was the head of a major biotech company, but the investigations of blogger Mia Garrett revealed that he had created a mutated virus with the goal of releasing it on the public to sell the cure for financial gain. To dispose of Mia, Thorne infected her with the mutated virus, leaving her to die in his office even after she came to him for the cure, disposing of her body in a local waste dump.

After Mia's body was taken to the Jeffersonian for analysis due to the rapid nature of decomposition caused by the virus, the needle that had been used to inject her also infected Arastoo Vaziri when he scratched his finger on the fragment still embedded in her shoulder. Despite a series of false leads including Mia's boyfriend Ben Carr, the subject of another article Byron Fuller, and Thorne's old colleague Doctor Tessa Burke, the team eventually identified Thorne as the source of the mutated virus based on other pathogens in it. Once Booth arrested Thorne, he took Thorne down to the Jeffersonian to not only show him Mia's remains, but for the rest of the team to make an emotional appeal to him to provide the antivirus so that they could save Arastoo's life. With Thorne still trying to cover his tracks as admitting the existence of the antivirus would be admitting his own guilt, he only confessed when Doctor Temperance Brennan pretended to inject him with a synthesized version of the virus so that he believed he had to give them the antidote to save his own life.

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