Experts say norovirus mutates every three years, and this winter, the U.S. is getting hit hard.
"Big alert from the CDC about this," said Dr. Richard Besser, ABC's chief medical editor. "Norovirus is the leading cause of diarrheal illness in the U.S. Every year, 21 million people get this. 800 people die from this."
Health officials say more than 140 outbreaks caused by the new Sydney strain of norovirus have been reported in the U.S. since September. Last month alone, the bug accounted for 58 percent of outbreaks of norovirus across the nation.
It was first identified last year in Australia, which is why it was dubbed the Sydney strain. It causes bouts of vomiting and diarrhea for a few days.
While the new strain may not be unusually dangerous, it's different, and many people might not be able to fight off its effects. The stomach virus has sickened people in Japan, Western Europe and other parts of the world.
"It comes on suddenly, typical symptoms of a stomach bug. Most people will do well, but the very young and the elderly, you have to worry about dehydration," Besser said.
Unlike the flu, the norovirus can't be spread by coughing or sneezing. The flu bug lives in the nose and throat, but the norovirus lives in the gut. Unlike salmonella and other food-borne illnesses, norovirus can also spread in the air, through droplets that fly when a sick person vomits.
"It stays in your gut, it lives in your gastrointestinal system, so it's in your vomit and it's in your stool when you have diarrhea," said infectious disease nurse Martha Mara
Mara adds the norovirus can also be spread through food and food preparation, so constant hand washing is of utmost importance. And use hand sanitizers properly.
"You're supposed to use friction to make sure that you're getting every surface of your hand, and just keep rubbing until the gel is absorbed and dry," she said.
It's also important to wipe down surfaces touched by contaminated vomit. Soap isn't enough. Mara says use a bleach-water mix or bleach wipes. Make sure to saturate the surface then let it dry. That's called the "kill time."
For those infected, there's really no medicine. They just have to ride it out for the day or two of severe symptoms, and guard against dehydration, experts said.
The Associated Press contributed to this report.