He is contacted by Agent Mulder to locate Dr. Baker, a Gregor being hunted by the alien bounty hunter.
Special Agent Weiss is able to locate Dr. Baker but meets his demise by being exposed to the corrosive green blood of the alien bounty hunter. He is found dead inside the trunk of his own car.
Later, in an autopsy room Scully reads through Weiss' autopsy report and finds his blood work shows polycythemia, an excessive production of red blood cells. She tells Mulder that Weiss' blood was "curdled like jelly, as if something cause the blood to thicken or clot prior to death."
When Mulder asks what could have caused it, Scully replies that a possible coagulating agent introduced into the body, but she tells Mulder there is no trace of anything in his blood. Mulder remembers what Chapel told him about the Gregors being able to contaminate the blood supply and he theorizes that a contaminant could have been used to kill agent Weiss. (TXF: "Colony")
Eventually, through Scully's field report, we learn that Weiss' cause of death was a thickening of the blood and it was believed that he was subjected to a virulent strain of virus whose origin and behavior are unexplained.
Scully later learns that the thickening of Weiss' blood was, in fact, a direct response to a retrovirus. (TXF: "End Game")