From Battlestar Wiki, the free, open content Battlestar Galactica encyclopedia and episode guide
Gauis Baltar is first introduced to Derrick when Jeanne visits him in Galactica's brig, where she asks him to bless her deathly ill child (Crossroads, Part I) who has viral encephalitis (He That Believeth In Me). Jeanne believes that such benediction would save her son ("Crossroads, Part I", "He That Believeth In Me").
Jeanne and Paulla Schaffer later extricate Derrick from Galactica's sickbay, bringing him back to the commune. Baltar tells the cultists that it is up to Derrick's immune system to fight off the illness; he adds that they must be strong for him and tells Jeanne that he prays for Derrick.
During that night, Baltar prays over Derrick, offering his life in return for Derrick's. The prayer moves the cultists, who are awakened by Baltar's praying. Following this prayer, Derrick's condition worsens, but then miraculously recovers after Baltar's assault in the officer's head by Charlie Connor. Upon Baltar's return to the commune, Derrick is alive and well, without a trace of fever (He That Believeth In Me).