Extremis has been referred to as a "virus" constantly since the story. The verbatim description offered by its inventor Maya Hansen, goes: "...Extremis is a super-soldier solution. It's a bio-electronics package, fitted into a few billion graphite nanotubes and suspended in a carrier fluid. A magic bullet, like the original super-soldier serum—all fitted into a single injection. It hacks the body's repair center—the part of the brain that keeps a complete blue print of the human body. When we're injured, we refer to that area of the brain to heal properly. Extremis rewrites the repair center. In the first stage, the body essentially becomes an open wound. The normal human blueprint is being replaced with the Extremis blueprint. The brain is being told the body is wrong. Extremis protocol dictates that the subject be placed on life support and intravenously fed nutrients at this point. For the next two or three days, the patient remains unconscious within a cocoon of scabs. (...) Extremis uses the nutrients and body mass to grow new organs. Better ones..."[10]
Effects of the Extremis process, apart from the changes specific to Tony Stark, included: greatly accelerated healing; immensely-boosted immune system; generation of "new, improved organs" (Tony's cardiovascular and respiratory systems were greatly upgraded); and, the side effect of increased aggressiveness.[11] Tony Stark had the super-powers removed from the Extremis compiler, though, effectively 'exchanging' them for the ability to interface directly with machines and his own armor; but, Mallen retained said super-powers, resulting in his high-level super-strength, super-speed, advanced invulnerability, and the abilities to breathe fire and project arcs of electricity from his hands, as well as being completely bullet-proof
