Activists release animals from a lab, ignoring warnings that they are infected with a highly contagious “rage” virus. 28 days later, almost everyone is gone. Jim (Cillian Murphy) wakes up in a hospital bed and wanders out into the deserted London streets.

This is a movie about a viral apocalypse. It has very scary zombies and jump-out-at-you attacks, but that is not what is most unsettling. Like the small group of survivors, we are cut off from any information and don’t know whether it is London, the UK, or the whole world that has been almost entirely wiped out. There is no way to know who or what to trust, no basis on which to evaluate alternative courses of action. When anyone becomes infected with the virus, there are only 10-20 seconds to kill him before he becomes a crazed, lethally infectious zombie. There is no time to plan to rebuild civilization. Survival is the only imperative.

And then, just as we begin to process — if not accept — all of that, the movie shifts into a whole other level of scariness. The zombies are terrifying, but they are not as bad as the “human” survivors, those people capable of higher reasoning and moral principles, and therefore of the most profound and disturbing betrayal.

Director Danny Boyle (“Trainspotting”) keeps the audience off-balance. Nightmarish quick cuts and digital video give the look of the movie a gritty, hallucinatory immediacy. Boyle also makes brilliant use of the empty artifacts, from the deserted London streets to a once-magnificent Gosford Park-style country house, now occupied by military, who eat rotting food around the table once used for glittering parties.

Each character gets just one defining quality (idealistic Jim, tough Selena, stout-hearted Frank, ingenue Hannah), but that just adds to the sense of urgency — we don’t have time to get to know them, just as they don’t have time to get to know each other.

Parents should know that this movie is very scary and deeply disturbing. It has has extreme and graphic peril and violence. Many characters are killed. Characters use very strong language. There is frontal male nudity. Characters drink and take drugs. There are sexual references, including rape.

Families who see this movie should talk about the different characters’ responses to ultimate questions about the meaning of life. Who is responsible for what happened? What will the world be like 10 years later? They should also talk about the enduring appeal of apocalyptic stories and the way they present moral choices in sharp relief.

Families who enjoy this movie will also enjoy other zombie-fests like the classic “Night of the Living Dead” and “Evil Dead,” and some of the other end-of-the-world movies like “On the Beach,” “The Road Warrior” trilogy, and the brilliant “12 Monkeys.”