A few weeks later, he served on the frontlines on Mar Sara at Fire Base Chimera. The zerg attacked the base and he and his allies fought back. During the battle, he was attacked and infected with the zerg hyper-evolutionary virus. The terrans did not understand what was happening to him and put him in a medical bay along with other wounded soldiers.
The zerg barged into the bay and slaughtered all of the patients except Madrid. They simply stared at him rather than attacking, and Madrid began to "hear" disturbing things in his mind which were urging him to kill.
The protoss Executor Tassadar entered the base, still intending to rescue the terrans. Madrid confronted Tassadar over the protoss attack on Chau Sara. Tassadar admitted that he destroyed it, but only because the zerg had secretly overrun the world, rendering the terrans incapable of resisting. Tassadar discovered that Madrid's infestation had gone too far to save him. Madrid requested a mercy killing, which Tassadar granted.
Metzen, Chris & Samuel Moore (March 29, 1999). "StarCraft: Revelations." Amazing Stories 596 (Spring): 20-27.