Jason Cahill was a CIA agent infected with a hallucinagen.
During their time in Amsterdam, both Nancy Cahill and her husband were infected with a powerful hallucinagen. Jason became feral and extremely hostile toward the environment and Nancy was forced to lock him in the closet of his house.
After Nancy shot and killed herself, APO sent Jack Bristow and Sydney to his house, where Sydney found the attic Jason was staying in. Inside, she found rotten food and several drawings of unholy symbols. Jason attacked her from behind and bit her on the throat, vomiting up blood and infecting her with the pathogen. Before he could do further damage, Jack arrived and shot Jason in the back.