Jason Cahill was a CIA agent infected with a hhallucinogen He was the husband of Nancy Cahill.
During their time in Amsterdam, both Nancy and Jason were infected with a powerful hallucinogen. Jason became feral and extremely hostile so Nancy was forced to lock him inside a secret passageway in their house.
After Nancy shot and killed herself, APO sent Jack and Sydney to his house, where Sydney found Jason. Inside the passageway, she found rotten food and several drawings of demonic figures and symbols. Jason attacked her from behind and bit her on the neck, vomiting up blood and infecting her with the pathogen in the process. Before he could do further damage, Jack arrived and shot Jason in the back.