Christian Fearon was an ill child patient, under the supervision of Doctor Dana Scully, while Scully was working as a Staff Physician in Our Lady of Sorrows Hospital in 2008. Christian suffered from a terminal illness which was undermining his brain function and Dr. Scully fought to save him against the wishes of the Hospital Administrator, Father Ybarra, a Catholic priest who was adamant to get the boy moved to a hospice for him to die in peace. The illness was identified as Sandhoff disease, also called Hexosaminidase A and B deficiency, an autosomal recessive lysosomal storage disorder that was known to affect lipid storage and cause progressive destruction of the central nervous system. It was clinically indistinguishable from Tay-Sachs disease, but affected two hexosaminidase enzymes.
Christian showed a great deal of courage as he faced imminent death, even after his parents were convinced by Father Ybarra to give up on him and let him die, a decision that was motivated by a belief - shared by Christian's parents and Father Ybarra - that they were acting in accordance with God's wishes. Dr. Scully informed Christian's parents that she was going ahead with stem cell treatment, which would cause great pain to the boy even though it did not reassure his complete recovery. Before the operation, Christian pointed out to Scully that she was now the person who looked scared. Dr. Scully later researched the best method of treatment for Christian and, as a byproduct of this research, Scully realized that Russian kidnappers, whom she had helped the FBI to investigate, had been and still were using their abductees for transplant operations. (The X-Files: I Want to Believe)
Christian Fearon was played by Marco Niccoli.
The original plot for The X-Files sequel film that would ultimately become The X-Files: I Want to Believe, conceptualized in 2003, included Scully working at a small family clinic and treating a young boy, but writers Chris Carter and Frank Spotnitz had not yet decided what direction they wanted to go with this plot thread. In this initial story concept, the boy's storyline ultimately converged with the storyline of Father Joe, so that Joe's storyline ended up being about the boy as much as it was about finding the missing women of the film's plot.
Frank Spotnitz realized that the boy storyline allowed himself and Carter to address the mythology of The X-Files television series, without the sequel film itself being a mythology story. Spotnitz believed that the boy storyline caused Scully (and Mulder, to a lesser extent) to wonder whether her refusal to accept the boy's diagnosis and the lack of a treatment was related to her own son – for whom the boy was possibly a substitute – and her own unresolved issues about having had to give William up for adoption. Spotnitz also thought that this factor of the film's plot had "great resonance" for anyone who was a fan of the series. (The Complete X-Files: Behind the Series, the Myths and the Movies)
Regarding casting this role, Carter specifically requested a boy who was aged between six and eleven and had cerebral palsy.  Many boys auditioned for the part, but Niccoli was ultimately cast in the role after having said that he wanted to be on IMDb. (The X-Files: I Want to Believe Blu-ray & DVD audio commentary) He was aged twelve at the time and, accompanied by his mother, he attended filming at Riverview Hospital, a sanatorium that stood in for Our Lady of Sorrows Hospital, for five long days over a two-week period in February 2008. Unusually, no makeup was applied to his skin for the role; this was because, according to his mother, Chris Carter said he had skin like porcelain.  His head was shaved, though, for the scenes in which Christian undergoes surgery, the final scenes of the movie that were filmed. (The X-Files: I Want to Believe Blu-ray & DVD audio commentary)