Many horror films use scary sounds to unnerve us, and “Contagion” — a horror film disguised as a medical thriller — is no exception. As the film begins, before we even see anything, we hear a cough. An ordinary-sounding cough, maybe. But it’s rather ominous when it’s the first sound in a movie called “Contagion.”

Steven Soderbergh directed it, his first feature since 2009’s larkish “The Informant!” He’s back to his no-nonsense approach now, determined to alarm and entertain in equal measure. The huge cast of characters and the global story covered from multiple angles hearken back to “Traffic”; like that film, “Contagion” is startlingly realistic and immaculately assembled, but lacking in heart.

The premise is simple and eerie. What if a new deadly infectious disease sprang up? How would the world’s governments react? How would ordinary citizens react? How many people would die before a vaccine could be produced? How long will you want to wash your hands after seeing this movie, especially considering movie theaters are the filthiest places on Earth? (Going to a movie theater to watch a film about germs is like going to a public library to read a book about homeless people.)

Our entry point to the story is a businesswoman, played by Gwyneth Paltrow, who returns to Minnesota from a trip to Hong Kong with basic flu-like symptoms that quickly escalate into something worse. It’s contagious, too, which is pretty bad news. Her husband (Matt Damon) and stepdaughter (Anna Jacoby-Heron) are the film’s constants after that, but the story expands to address almost every conceivable aspect of the epidemic. At the Centers for Disease Control, Laurence Fishburne heads up the effort to identify and treat the virus, with Demetri Martin and Jennifer Ehle testing vaccines as they are formulated. The CDC’s Kate Winslet goes to Minnesota to investigate and is met with resistance by the state’s health department, mostly because of budgetary concerns and the sense that we’ve had false alarms before. (The recent H1N1 non-epidemic is mentioned.) The World Health Organization’s Marion Cotillard goes to Hong Kong to track down the disease’s origins.

Meanwhile, it is spreading. As more days go by without a vaccine, people begin to panic, hoard, and loot. Jude Law turns up as a muckraking blogger and conspiracy theorist pushing a holistic cure that he says the government doesn’t want you to know about; Elliott Gould plays a scientist assisting the CDC; there’s Bryan Cranston as a U.S. military official, Enrico Colantoni as a government official, many people with less recognizable names and faces playing a host of other necessary figures.

Through it all, Soderbergh keeps us nervous with constant, subtle reminders of where the danger lurks, i.e., everywhere. A bowl of peanuts at a bar, a credit card passed to a cashier, the door handle in a public building — everything is a potential threat, and Soderbergh (who serves as his own cinematographer) lingers over such images just long enough to make his point.

What’s scariest about “Contagion” is that every part of it could really happen. No suspension of disbelief is required here. Furthermore, while there are well-known protections against vampires and zombies (and easy ways to tell if somebody is one), the only way to avoid a deadly virus is to avoid all human contact. We must also grapple with the fact that if an outbreak like this occurred in real life, there would indeed be people seeking to cash in on it, and bureaucratic inefficiency causing delays, and fearmongers giving counterproductive advice. People would rely on the government to help them while simultaneously not trusting it to do so. In short, “Contagion” is effective because it plays not on our fears of fictional boogeymen and monsters, but on actual threats.

It’s far less effective when it tries to show the human side of things. The largely plot-driven screenplay, by Scott Z. Burns (who adapted “The Informant!”), makes an effort now and then to engage our emotions, but it seldom succeeds. Moments like Laurence Fishburne interacting with a lowly CDC janitor (John Hawkes) come off as trite, while Matt Damon’s efforts to protect his daughter don’t register as anything more than perfunctory. Some films have managed to tell a global story while maintaining an emotional center, but this isn’t one of them. Maybe it’s just as well, though. You don’t want to get attached to people in a movie that will make you never want to touch anyone again.

B (1 hr., 45 min.; PG-13, one F-word, moderate profanity, some grisly images, mild violence.)