Topher Grace says he signed on to star in “The Hot Zone” because he “loved the book,” adding: “It was the first book that just totally ... it scared the s--t out of me.”

The National Geographic Channel’s six-hour adaptation of Richard Preston’s 1994 nonfiction thriller — full title: “The Hot Zone: A Terrifying True Story” — revolves around a 1989 incident when a mutated form of the Ebola virus was found at a primate facility in Reston, Va., a suburb of Washington, D.C.

“It was set up to be the biggest catastrophe ever in the United States,” said executive producer Kelly Souders. “Had things not broken slightly here and there, it would’ve been a disaster.”

Three decades later, there’s still no cure for Ebola. And it’s 90% fatal.

“It might, to this day, be the scariest thing I ever read,” said Grace, best known for his starring role in “That ’70s Show.” “Because it was real. This isn’t a fictional monster that’s chasing us. It’s real. It’s still alive. And also it’s invisible.”

He stars as physician Peter Jahrling, a virologist who fears he has contracted the disease. “The Hot Zone” revolves around Lt. Col. Nancy Jaax (Julianna Margulies), who’s the chief pathologist at the U.S. Army Medical Institute of Infectious Diseases. (The name has been changed, but the character is based on a real person.)

Jaax is tough and no-nonsense, even when she’s being mansplained to by people who know far less than she does — who don’t take the threat as seriously as she does.

“I looked at her character as a hero, because she really emphasized what a threat this was and got the ball rolling to stop it from spreading,” said Margulies (“The Good Wife,” “ER”). “But to her, it was just another day at the office.”

Another day at the office where you know that it’s entirely possible that nine out of 10 Americans — of the world’s population — could be dead soon if you don’t do your job. This is a six-hour horror/disaster movie.

(National Geographic | Amanda Matlovich) Dr. Peter Jahrling (Topher Grace), a civilian virologist, looks through the microscope eyepiece as Dr. Nancy Jaax (Julianna Margulies) and Ben Gellis (Paul James) look on.

(The six hourlong episodes air in two-hour blocks Monday-Wednesday at 7 p.m. MDT on NGC. The cast includes Noah Emmerich as Jaax’s husband; Liam Cunningham as a reclusive Ebola expert; James D’Arcy as a consultant for the Centers for Disease Control; Robert Sean Leonard as a corporate flunky; Paul James as a lab tech; and Nick Searcy as a janitor at the infected facility.)

And, yes, “The Hot Zone” is more than a little gross at times. Ebola is a disgusting disease, with symptoms that include bleeding, vomiting and diarrhea — a near-certain death that was incubating in a major American population center.

“Everyone thinks that because Ebola is found in these faraway African villages that it has nothing to do with us here in the U.S.,” Margulies said. “And to see something that happened in 1989 — to see that Ebola touched U.S. soil — basically, we dodged a bullet.”

Thirty years later, “people understand this is something we should take seriously, but I don’t see any action,” Margulies said.

That’s really scary.