Health networks are seeing abnormally high cases of the stomach bug that’s taking a large number of people out of action and leaving them stuck in the bathroom.

Nationwide, outbreaks of norovirus, which causes vomiting and diarrhea, have surged to a level higher than they’ve been for more than a decade, according to the Centers for Disease Control and Prevention’s NoroSTAT data.

Though norovirus is often also called the “stomach flu,” it is not an influenza virus or even a respiratory infection; it is a gastrointestinal infection, the most common one at that. The primary symptoms of a norovirus infection include vomiting, diarrhea, stomach pain and dehydration, though headache, fever and body aches are also possible during an infection. The virus incubates in the body for 24 to 48 hours and symptoms usually last one to three days.

Norovirus outbreaks occur most frequently during the winter months and are most common in congregate settings such as schools, military barracks, nursing homes, cruise ships and, of course, hospitals, where it can spread from patients to staff, Dr. Jeffrey Jahre, St. Luke University Health Network’s chief emeritus of infectious diseases, said.

“The problem here is that not only do the patients get it, but the staff do too, and that puts a terrific strain on everybody that’s involved,” Jahre said.

The cause of the increased spread this year is believed partly to be due to a strain known as GII.17[P17] that has not been widely prevalent for several years, as well as people increasingly gathering together, Jahre said.

Dr. Mark Knouse, chief of Lehigh Valley Health Network’s Division of Infectious Disease, said another reason why norovirus cases may be more prevalent this year is due to how many variants of norovirus are circulating right now.

“There’s like 40-plus genotypes of this virus and many of them can cause disease, and they change frequently, just like you’ve heard about respiratory viruses, this virus changes often,” Knouse said. “It seems that we get immune to maybe one or two of the strains, yet another strain pops up and that’s one of the reasons … as to why it’s so bad this last season.”

He said when you combine the above factors with the fact norovirus can stick to surfaces for days or weeks, and someone only has to come in contact with a small amount of the virus to get infected, you get the results of the current norovirus season.

Cases of norovirus are rarely fatal, resulting in only about 900 deaths annually in the U.S. per year, primarily among adults 65 or older, according to the CDC. However, it does cause an average of 109,000 hospitalizations and 465,000 emergency room visits each year. In severe cases, dehydration caused by a norovirus infection can lead to hospitalization, particularly in those who have compromised immune systems or underlying heart or kidney disease.

“Although the vast majority of people never get admitted with norovirus, there are people who do because it’s very difficult to maintain hydration when you literally lose total taste or appetite to keep anything down. You pretty much have to almost force yourself to take small sips of water or other kinds of liquids, because if you drink too much, you’re going to throw up,” Jahre said.

Knouse said if severe dehydration is ignored it can lead to abnormalities in the body and even kidney injury.

Jahre said the severity isn’t the main problem for most people though. For them the issue is that the disease takes people out of action, whether it be from work or school, and leaves them feeling terrible for days, and infectious to those around them for potentially weeks. The CDC estimates each year that worldwide norovirus infections have an economic impact of $60 billion due to health care costs and lost productivity.

Norovirus doesn’t affect everyone uniformly; some people are particularly resistant to norovirus infections, while others will experience the full suite of symptoms. Whether or not you are likely to get sick is believed to be related to blood type: People with B-type blood seem to be resistant to certain strains of norovirus. However, this quality may not be as useful this year.

“It seems likely there is enough strain circulating that no one can rely on blood type to protect them,” Knouse said.

The virus can be acquired from numerous sources. Human-to-human transmission or contact with surfaces touched by sick people is a common vector but food is another common culprit. Norovirus can exist in or on unwashed leafy green vegetables or fresh fruits that have residual infected fecal matter on them, or in uncooked filter feeding shellfish such as clams and oysters. Since June the U.S. Food and Drug Administration has issued several advisories to restaurants and retailers to not serve oysters from certain sources in South Korea, British Columbia and Washington state.

Preventing infection can be difficult if you are in a space with someone who has a case of norovirus, as the virus clings to surfaces, sometimes up to weeks, and hand sanitizers are ineffective against it, Jahre said. Even being recently infected with a case of norovirus is not necessarily a good source of immunity to the illness, as the virus mutates frequently and there tend to be many variants in circulation at a given time.