Norovirus

Norovirus is a highly contagious virus that causes gastroenteritis, which is an inflammation of the stomach and intestines. It is a major global health concern, causing approximately 685 million cases and 200,000 deaths every year. Because it often spreads more easily during the colder months, people sometimes call it the "winter vomiting disease." In the United States, norovirus is responsible for about half of all foodborne disease outbreaks. While most people recover quickly, the virus can be dangerous for certain groups. These include young children, the elderly, and people with weakened immune systems.

The virus is a member of the Caliciviridae family. It is classified as a non-enveloped, single-stranded, positive-sense RNA virus. This means the virus does not have a fatty outer layer, or envelope, surrounding its genetic material. Because it lacks this envelope, many alcohol-based hand sanitizers are ineffective at killing it. The virus structure is icosahedral, which is a shape with many flat sides. These capsids can range in diameter from 23 to 40 nanometers. Larger capsids show T=3 symmetry and contain 180 VP1 proteins. Smaller capsids show T=1 symmetry and contain 60 VP1 proteins.

Once a person is exposed, the virus begins a specific replication cycle inside the host's cells. The process starts when the virus attaches to host receptors in the intestinal epithelium. This attachment allows the virus to enter the cell through a process called endocytosis. Once inside, the virus replicates within the cytoplasm, which is the fluid inside the cell. The virus uses a method called positive-stranded RNA transcription to make copies of itself. It also uses translation to create its necessary proteins. After the new viruses are assembled, the host cell undergoes lysis, or breaking open, to release the new particles.

Norovirus is primarily spread through the fecal-oral route. This occurs when the virus moves from infected waste into another person's mouth. Transmission can be indirect through contaminated food or water. Shellfish and salad ingredients are often linked to these outbreaks. For example, eating shellfish that has not been heated enough is a high risk. Transmission can also be direct through person-to-person contact. In some cases, the virus can become aerosolized. This happens when someone vomits, sending tiny particles into the air. People can then breathe in these particles or touch contaminated surfaces.

Researchers have studied how the virus spreads in crowded environments. In one 1998 incident in a restaurant, one person vomited on the floor. Even though staff cleaned up quickly, many people became ill three days later. More than 90% of the people at the same table as the sick person fell ill. There was a clear correlation between the risk of infection and how close a person sat to the original site. Another outbreak at a scout jamboree in the Netherlands showed how contagious the virus is. One infected person spread the virus to an average of 14 other people. Even after better hygiene was introduced, each sick person still infected about 2.1 others.

Scientists categorize noroviruses into different genetic groups. There are at least seven known genogroups, labeled GI through GVII. Most human infections come from genogroup I (GI) and genogroup II (GII). The Norwalk virus is a member of the GI group. Many adult outbreaks are caused by the GII.4 genotype, which can spread globally. These viruses evolve very quickly. The mutation rate of norovirus is very high compared to other RNA viruses. This fast evolution helps the virus change and spread through different populations.

Symptoms of infection usually appear about 24 hours after exposure. Common signs include nausea, forceful vomiting, watery diarrhea, and abdominal pain. Some people may also experience fever, headaches, or muscle aches. Most people feel better within one to three days. However, the virus can cause serious complications like dehydration. This is especially risky for people who cannot keep fluids down. There is no specific medicine or vaccine to treat norovirus. Doctors instead use supportive care, which focuses on replacing lost fluids. Oral rehydration solutions are the best way to manage these symptoms.