Gordon Amherst is the individual behind the Green Poison, or Dollar Flu as some NPCs refer to it. His whereabouts were unknown until the end of the game when he is found dead in an empty laboratory. If the player character uses a scope to look at his face, they will notice that the scientist was probably killed by his own disease, as there are pimples all over his face.
Jessica Kandel had contempt for him prior to the outbreak, and some of his viewpoints and speeches nearly caused a riot at a campus.
His explanation for creating and spreading the Green Poison was that humanity was too powerful on a global scale, mother nature needed help in "advancing her creations", that humans eventually needed to be put in check, and he was the cure for it. Furthermore, he is also a believer in the concept of natural selection that the strong are the only ones that deserve to live, not the weak, and his creation is the tool to ensure that concept happens since the strong will survive the virus while the weak cannot. With the breakthrough of digitizing DNA, the process of growing a generation of lab-grade pathogens could be done a thousand times faster than conventional methods. Intrigued by this, he obtained smallpox strains from Vitaly Tchernenko and utilized a 3D biological printer to create Green Poison. He ironically died to his own creation, which he infected himself with to live up to his view point of natural selection.
Discovery of involvement by CERA
It is not revealed in-game how the suspicion of developing the Green Poison came to rest on dr. Amherst in the first place, apart from Faye Lau mentioning (during Jessica Kandel's introduction to the Base of Operations, after the mission Madison Field Hospital) that CERA 'is pretty sure' about it.
Faye Lau: "Here's a good place to start. CERA is pretty sure a Dr. Gordon Amherst had something to do with the outbreak."From that point on, dr. Amherst's involvement in the outbreak is treated as a certainty. During the introduction to the next mission where Jessica Kandel appears, Broadway Emporium, she has this to say:
Jessica Kandel: "God, that asshole. Saw him present a paper at Columbia once, he nearly started a riot. But if he's part of this, I need to talk to him."
Jessica Kandel: "So that little weasel picked a bunch of department stores to spread his little Black Friday Surprise. And everyone knows Abel's - it might even be Amherst's ground zero. (...)"