“Contagion” may be the most unexpectedly terrifying movie of the year. It’s not exactly a horror movie in the traditional sense of the word, since there are no axe murderers, ghosts or zombies, but its step-by-step breakdown of the spread of a potentially apocalyptic virus is every bit as unnerving as anything George Romero or John Carpenter has ever accomplished.

The film doesn’t waste a minute, introducing Beth (Gwyneth Paltrow), who’s returning from a business trip overseas, and promptly making her one of the first victims of the deadly virus that functions as the film’s villain. From there, “Contagion” takes on a vaguely novelistic structure, introducing the key figures trying to prevent the epidemic and then slowly toppling their every effort. Scott Z. Burns’ script moves deliberately, laying out each government agency and their different actions, never portraying them as right or wrong decisions, but as the acts of people trying to do their best in the situation from hell.

Director Steven Soderbergh has been publicly threatening to retire, and “Contagion” makes it crystal clear what a shame that would be. Soderbergh builds relentless tension throughout the film, weaving in small moments of human decency amongst the large-scale apocalyptic material. It’s refreshing how effortlessly Soderbergh juggles a dozen major characters, keeping the film moving quickly and not feeling the need to have every single storyline intersect and overlap with another, like so many large ensemble dramas.

Every member of the film’s sprawling cast pulls their weight. Matt Damon shines as Mitch, husband to Paltrow’s Beth, and his helpless terror at the plight he and what remains of his family are in is contagious. As an epidemic specialist, Kate Winslet starts off as authoritative and imposing, and Winslet makes her characters’ slow deterioration hurt us just as much as it does her. As a self-righteous blogger, Jude Law is shamelessly smarmy, but his character never quite develops into a very compelling figure. Marion Cotillard is the film’s weakest link, playing another medical specialist whose character arc makes some pretty huge leaps that the screenplay can’t quite justify.

Thanks to its matter-of-fact structure, speedy pace and mostly strong performances, “Contagion” is a reliable, often chilling thriller. But more so, it’s a film that worms its way under your skin in some very subtle ways. It makes you extremely aware every time you touch your face, question every hand you shake and sends a chill down your spine every time you hear a cough or a sneeze. If that’s not the mark of a truly exceptional thriller, what is?