[Read: Whatever happened to toilet plumes?]

Norovirus Is Almost Impossible to Stop

“Bleach is my friend right now,” says Annette Cameron, a pediatrician at Yale School of Medicine, who spent the first half of this week spraying and sloshing the potent chemical all over her home. It’s one of the few tools she has to combat norovirus, the nasty gut pathogen that her 15-year-old son was recently shedding in gobs.

Right now, hordes of people in the Northern Hemisphere are in a similarly crummy situation. In recent weeks, norovirus has seeded outbreaks in several countries, including the United Kingdom, Canada, and the United States. Last week, the U.K. Health Security Agency announced that laboratory reports of the virus had risen to levels 66 percent higher than what’s typical this time of year. Especially hard-hit are Brits 65 and older, who are falling ill at rates that “haven’t been seen in over a decade.”

Americans could be heading into a rough stretch themselves, Caitlin Rivers, an infectious-disease epidemiologist at Johns Hopkins University, told me, given how closely the U.S.’s epidemiological patterns tend to follow those of the U.K. “It does seem like there’s a burst of activity right now,” says Nihal Altan-Bonnet, a norovirus researcher at the National Institutes of Health. At her own practice, Cameron has been seeing the number of vomiting and diarrhea cases among her patients steadily tick up. (Other pathogens can cause gastrointestinal symptoms as well, but norovirus is the most common cause of foodborne illness in the United States.)

To be clear, this is more a nauseating nuisance than a public-health crisis. In most people, norovirus triggers, at most, a few miserable days of GI distress that can include vomiting, diarrhea, and fevers, then resolves on its own; the keys are to stay hydrated and avoid spreading it to anyone vulnerable—little kids, older adults, the immunocompromised. The U.S. logs fewer than 1,000 annual deaths out of millions of documented cases. In other high-income countries, too, severe outcomes are very rare, though the virus is far more deadly in parts of the world with limited access to sanitation and potable water.

Still, fighting norovirus isn’t easy, as plenty of parents can attest. The pathogen, which prompts the body to expel infectious material from both ends of the digestive tract, is seriously gross and frustratingly hardy. Even the old COVID standby, a spritz of hand sanitizer, doesn’t work against it—the virus is encased in a tough protein shell that makes it insensitive to alcohol. Some have estimated that ingesting as few as 18 infectious units of virus can be enough to sicken someone, “and normally, what’s getting shed is in the billions,” says Megan Baldridge, a virologist and immunologist at Washington University in St. Louis. At an extreme, a single gram of feces—roughly the heft of a jelly bean—could contain as many as 5.5 billion infectious doses, enough to send the entire population of Eurasia sprinting for the toilet.

Unlike flu and RSV, two other pathogens that have bounced back to prominence in recent months, norovirus mainly targets the gut, and spreads especially well when people swallow viral particles that have been released in someone else’s vomit or stool. (Despite its “stomach flu” nickname, norovirus is not a flu virus.) But direct contact with those substances, or the food or water they contaminate, may not even be necessary: Sometimes people vomit with such force that the virus gets aerosolized; toilets, especially lidless ones, can send out plumes of infection like an Air Wick from hell. And Altan-Bonnet’s team has found that saliva may be an unappreciated reservoir for norovirus, at least in laboratory animals. If the spittle finding holds for humans, then talking, singing, and laughing in close proximity could be risky too.

[Read: Whatever happened to toilet plumes?]

Once emitted into the environment, norovirus particles can persist on surfaces for days—making frequent hand-washing and surface disinfection key measures to prevent spread, says Ibukun Kalu, a pediatric infectious-disease specialist at Duke University. Handshakes and shared meals tend to get dicey during outbreaks, along with frequently touched items such as utensils, door handles, and phones. One 2012 study pointed to a woven plastic grocery bag as the source of a small outbreak among a group of teenage soccer players; the bag had just been sitting in a bathroom used by one of the girls when she fell sick the night before.

Once a norovirus transmission chain begins, it can be very difficult to break. The virus can spread before symptoms start, and then for more than a week after they resolve. To make matters worse, immunity to the virus tends to be short-lived, lasting just a few months even against a genetically identical strain, Baldridge told me.

Day cares, cruise ships, schools, restaurants, military training camps, prisons, and long-term-care facilities can be common venues for norovirus spread. “I did research with the Navy, and it just goes through like wildfire,” often sickening more than half the people on tightly packed ships, says Robert Frenck, the director of the Vaccine Research Center at Cincinnati Children’s Hospital. Households, too, are highly susceptible to spread: Once the virus arrives, the entire family is almost sure to be infected. Baldridge, who has two young children, told me that her household has weathered at least four bouts of norovirus in the past several years.

(A pause for some irony: In spite of norovirus’s infectiousness, scientists did not succeed in culturing it in labs until just a few years ago, after nearly half a century of research. When researchers design challenge trials to, say, test new vaccines, they still need to dose volunteers with norovirus that’s been extracted from patient stool, a gnarly practice that’s been around for more than 50 years.)

Norovirus spread doesn’t have to be a foregone conclusion. Some people do get lucky: Roughly 20 percent of European populations, for instance, are genetically resistant to common norovirus strains. “So you can hope,” Frenck told me. For the rest of us, it comes down to hygiene. Altan-Bonnet recommends diligent hand-washing, plus masking to ward off droplet-borne virus. Sick people should isolate themselves if they can. “And keep your saliva to yourself,” she told me.

[Read: The stomach-flu mystery]

Rivers and Cameron have both managed to halt the virus in their homes in the past; Cameron may have pulled it off again this week. The family fastidiously scrubbed their hands with hot water and soap, donned disposable gloves when touching shared surfaces, and took advantage of the virus’s susceptibility to harsh chemicals and heat. When her son threw up on the floor, Cameron sprayed it down with bleach; when he vomited on his quilt, she blasted it twice in the washing machine on the sanitizing setting, then put it through the dryer at a super high temp. Now a couple of days out from the end of their son’s sickness, Cameron and her husband appear to have escaped unscathed.

Norovirus isn’t new, and this won’t be the last time it hits. In a lot of ways, “this is back to basics,” says Samina Bhumbra, the medical director of infection prevention at Riley Children’s Hospital. After three years of COVID, the world has gotten used to thinking about infections in terms of airways. “We need to recalibrate,” Bhumbra told me, “and remember that other things exist.”