Scully discovers a virulent strain of virus which caused Agent Weiss' blood to thicken. In talking to another naval doctor, she learns that the virus is a retrovirus and when the body is exposed to the virus, the body produces a massive amount of red blood cells. The doctor was able to control the growth of the retrovirus by subjecting it to cold temperatures, in essence, making the virus dormant. (TXF: "End Game")