It floats in the air and it sticks to dishes. A quick rinse under the faucet doesn't remove it from your hands, and people can spread it before they feel sick themselves.
No wonder norovirus makes 21 million people sick every year in the United States.
Health officials in Boston now say they believe norovirus is what made scores of Boston College students sick after they ate at a nearby Chipotle restaurant. The school says 120 students are ill.
Because Chipotle's been linked to an outbreak of E. coli that infected 52 people in nine states in recent weeks, the immediate assumption was that this was part of the E. coli outbreak.
Both infections are most commonly spread in food, they originate in feces, and they both cause diarrhea and stomach cramps.
But there are some differences between E. coli and norovirus:
Norovirus acts quickly
Symptoms can come on 12 to 48 hours after exposure. E. coli usually starts causing symptoms a little more slowly, usually three to four days after it gets into the body, although sometimes it makes people sick a day later.
Norovirus makes you vomit, violently
Norovirus is commonly known as stomach flu or winter vomiting disease. Both infections cause abdominal cramps and diarrhea, and often fever. E. coli infections can cause bloody diarrhea and sometimes but not always vomiting; norovirus is more likely to cause watery diarrhea.
High numbers point to norovirus
When norovirus hits, it often causes huge outbreaks — for instance, 700 people sickened on the Royal Caribbean ship Explorer of the Seas in January 2014 or 142 people infected on a Royal Norwegian cruise ship. E. coli outbreaks often play out over a longer period of time and the numbers reported are usually much smaller. For instance, 19 have been reported sick since November 23 in the outbreak linked to celery sold at Costco, 7-11 and other stores.
Each year, norovirus makes 70,000 people sick enough to go to the hospital. As many as 800 people die, mostly elderly patients who become dehydrated. It's the most common cause of foodborne-disease outbreaks, CDC says.
Researchers who put together a "vomiting machine" demonstrated one reason why earlier this year. The violent vomiting that marks norovirus sends small particles of that vomit into the air, carrying pieces of virus.
"We think that there's a at least a million particles released in a vomiting event and maybe more," said Lee-Ann Jaykus, the N.C. State food science professor who leads that research.
People can be infected with as few as 20 to 1,300 microscopic viral particles, so their study shows that vomiting could indeed spread the infection through the air. And the findings help explain how a batch of teenaged soccer players got sick from eating packaged cookies in Washington state in 2010.
One of the girls had thrown up in the hotel bathroom, spreading an aerosol of norovirus that landed everywhere, including on a reusable grocery bag hanging in the room. The bag, which later was used to carry the cookies, tested positive for the bug two weeks later.
Norovirus is hard to wash away. Studies show a quick application of hand sanitizer won't get rid of it, and most people don't wash their hands properly, either — it takes about 30 seconds of vigorous rubbing using hot water and soap to wash away the tiny bits of virus, and that means getting under the nails, too.
"Imagine you have a food handler who uses the bathroom and they haven't washed their hands thoroughly," Allison Aiello, who studies how disease spreads at the University of North Carolina, says. "They can end up preparing a salad for the diners that evening and end up infecting a lot of people because the food isn't cooked."