Norovirus is the most common cause of acute gastroenteritis, or tummy trouble, in the United States, making 21 million people sick annually. The virus — which got its name from Norwalk, Ohio, where it was first isolated in a school during a 1968 outbreak — causes viral gastroenteritis, a nonbacterial inflammation of the digestive tract and small intestines. (Oddly enough, it doesn’t appear to affect the large intestines.)

The virus enters the body orally and travels to the small intestine through the stomach. Some research suggests there is a connection between rate of norovirus infection and a person’s blood type.

Blood type is determined by genes we inherit from our parents (genotype) and how an individual displays a certain trait (phenotype.) Individuals with type O phenotype were found to be most susceptible to norovirus infection. Those with B and AB phenotypes were at decreased risk of infection. When these B and AB phenotypes were infected, they were more likely to be asymptomatic. 

The virus may particularly like people with the blood group O because those with type O have a receptor present in saliva that the virus may more easily attach itself to it.

If you have a blood type other than O, you can still contract norovirus, but the disease may not be as severe, according to Dr. Gerald Evans, a professor of medicine, biomedical and molecular sciences at Queen's University. Evans is also the medical director for infection prevention and control at Kingston General Hospital.

However , a 2010 study from the Centers of Disease Control and Prevention found no protections for people with type B blood. The study found the virus infected blood types across the board, suggesting no protection for any particular type.

Complicating resistance to infection by norovirus is that the virus appears to continually mutate, changing to avoid destruction by our immune system, similar to the flu virus. Most people don't achieve long-term immunity to the virus and repeated infections are common. In 2012, a new strain was detected in Australia called GII.4 Sydney, and people in the United States and other countries have already been infected with this new strain.

Sadly, there’s no real treatment for it except for waiting it out and trying home comfort and natural remedies. There’s no vaccine, and recent studies show it’s one of the hardest viruses to ditch. Symptoms — generally abdominal cramps, vomiting and diarrhea, fever, malaise and muscle aches — appear 24 to 48 hours after exposure, and last from 12-48 hours. Serious illness is rare, and usually only appears in young children, the elderly or those with compromised immune systems. While healthy people can recover from the virus after a couple of exhausting days, the virus can hang around in people with weak immune systems for months.

After a person has a bout of norovirus infection, they are immune, but immunity is temporary – only about 14 weeks. People can get norovirus illness many times during their lifetime. Outbreaks affect people of all ages and in a variety of settings, though nursing homes, cruise ships and schools are popular venues. But anywhere large populations are crowded together is fodder for norovirus to thrive. The CDC says most outbreaks are caused when norovirus is spread from infected people to others. But, the virus can also spread by consuming contaminated food or water and touching things that have the virus on them. 

Worse, norovirus can live on surfaces for unknown periods of time. Simple cleaning alone doesn’t always kill it, and it takes just a few particles of virus to sicken someone. A team at Ohio State University found the virus stuck to plates that had been washed in restaurant-like conditions — and they found sticky dairy products like cheese helped the virus adhere. 

Diligent hand washing is still the best and only defense against norovirus particles, which can be ushered into your mouth through your hands. Thorough cleaning with a bleach solution is recommended if a family member has been ill with norovirus symptoms. Eat from reputable, clean eateries and avoid raw shellfish, salad bars or foods that were prepared by unknown entities.

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