(ABC News) -- A new strain of norovirus that wreaks havoc on people's stomachs is so vicious that it's being called a "superbug" by doctors.
Though it was first identified in Australia, this norovirus - also called the Sydney strain - is quickly spreading across the United States.
In an average year, according to the Centers for Disease Control and Prevention, about 21 million Americans get the norovirus, with classic stomach flu symptoms such as vomiting and diarrhea.
Eight hundred die. Symptoms come on very suddenly, within hours after a person has been exposed to it.
Because no one has immunity to this new strain, more Americans - perhaps 50 percent more, the CDC says - could become violently ill.
While the flu is spread mostly in the air by sneezes and coughs and a person needs to breathe in as many as 1,000 virus particles to get sick, the norovirus is far more contagious. Just 18 norovirus particles can make a person sick.
The flu can last two to four hours on hard surfaces outside your body, but the norovirus can survive and remain infectious for weeks.
To keep the norovirus away, medical experts suggest cleaning the house with bleach, not just regular detergents. They also say that while hand sanitizers kill the flu virus, they are not effective at getting rid of the norovirus.
Medical experts suggest washing your hands with soap and water repeatedly to keep the norovirus off them.