SPOILER WARNING: This article contains major spoilers for "The Icicle Cometh," the latest episode of The Flash.
Caitlin Snow isn't a metahuman after all. In "The Icicle Cometh," the latest episode of The Flash, her father Thomas revealed the truth behind her powers and dual personality. When she was a child, he used experimental cryogenic gene therapy on her in an effort to prevent her from developing ALS, a fatal disease she inherited from him. It cured her, but the side effects caused her Killer Frost persona to manifest.
"ALS is a horrible disease and I grew desperate and I began experimenting with therapies using cryogenics," Thomas explained to Team Flash. "The experiments became more and more dangerous, and I quarantined myself for the sake of everyone's safety, but I did manage to freeze the progression of that disease, but not before the site was entirely closed down and I have been here every since."
"I cured my ALS, but I created something even more deadly. Every time my emotions spike, my malignancy spreads," he lamented. "I wish I had just left well enough alone, for both of us."
"Wait, did I have the genetic marker for ALS too? And you tried to cure me?" Caitlin demanded to know. "So my powers came from you? Killer Frost is my cure?"
"I wasn't going to let the ALS manifest in you like it did in me," Thomas admitted.
"So you used experimental cryogenic gene therapy on your daughter," Cisco pointed out.
"Caity, I made alterations to the treatment. I tried to make it safer," Thomas said. "You started to exhibit dual personalities soon after I treated you."
As such, Caitlin's powers did not stem from the particle accelerator explosion, which gave both Barry and Cisco their abilities. This puts her -- and her father -- into a category all of their own. This also explains why Carla Tannhauser, Caitlin's mother, lied about Thomas' death; it seems she was trying to protect her daughter from Thomas' experiments and the painful truth behind her powers.
Now that Caitlin knows the truth, it will have lasting ramifications for her. "One of my favorite things about this season is that there really seems to be a father/daughter theme, and those relationships sort of exist over several different storylines," Caitlin actor Danielle Panabaker told CBR at the red carpet event for The Flash's 100th episode. "Yes, her father does leave, but his impact is lasting and I'm certain that's not the last we see of Caitlin's dad."
Airing Tuesdays at 8 p.m. ET/PT on The CW, The Flash stars Grant Gustin, Candice Patton, Carlos Valdes, Danielle Panabaker, Tom Cavanaugh, Jesse L. Martin, Danielle Nicolet, Hartley Sawyer, and Jessica Parker Kennedy.