CDC Answers Your Questions About

Noroviruses: Q&A

What are noroviruses?

Noroviruses are a group of viruses that cause the "stomach flu," or gastroenteritis (GAS-tro-enter-I-tis), in people. The term norovirus was recently approved as the official name for this group of viruses. Several other names have been used for noroviruses, including:

• Norwalk-like viruses (NLVs)
• caliciviruses (because they belong to the virus family Caliciviridae)
• small round structured viruses.

Viruses are very different from bacteria and parasites, some of which can cause illnesses similar to norvirus infection. Viruses are much smaller, are not affected by treatment with antibiotics, and cannot grow outside of a person's body.

What are the symptoms of illness caused by noroviruses?

The symptoms of norovirus illness usually include nausea, vomiting, diarrhea, and some stomach cramping. Sometimes people additionally have a low-grade fever, chills, headache, muscle aches, and a general sense of tiredness. The illness often begins suddenly, and the infected person may feel very sick. The illness is usually brief, with symptoms lasting only about 1 or 2 days. In general, children experience more vomiting than adults. Most people with norovirus illness have both of these symptoms.

What is the name of the illness caused by noroviruses?

Illness caused by norovirus infection has several names, including:

• stomach flu - this "stomach flu" is not related to the flu (or influenza), which is a respiratory illness caused by influenza virus.
• viral gastroenteritis - the most common name for illness caused by norovirus. Gastroenteritis refers to an inflammation of the stomach and intestines.
• acute gastroenteritis
• non-bacterial gastroenteritis
• food poisoning (although there are other causes of food poisoning)
• calicivirus infection

How serious is norovirus disease?

Norovirus disease is usually not serious, although people may feel very sick and vomit many times a day. Most people get better within 1 or 2 days, and they have no long-term health effects related to their illness. However, sometimes people are unable to drink enough liquids to replace the liquids they lost because of vomiting and diarrhea. These persons can become dehydrated and may need special medical attention. This problem with dehydration is usually only seen among the very young, the elderly, and persons with weakened immune systems. There is no evidence to suggest that an infected person can become a long-term carrier of norovirus.

How do people become infected with noroviruses?

Noroviruses are found in the stool or vomit of infected people. People can become infected with the virus in several ways, including:

• eating food or drinking liquids that are contaminated with norovirus;
• touching surfaces or objects contaminated with norovirus, and then placing their hand in their mouth;
• having direct contact with another person who is infected and showing symptoms (for example, when caring for someone with illness, or sharing foods or eating utensils with someone who is ill).

Persons working in day-care centers or nursing homes should pay special attention to children or residents who have norovirus illness. This virus is very contagious and can spread rapidly throughout such environments.

When do symptoms appear?

Symptoms of norovirus illness usually begin about 24 to 48 hours after ingestion of the virus, but they can appear as early as 12 hours after exposure.

Are noroviruses contagious?

Noroviruses are very contagious and can spread easily from person to person. Both stool and vomit are infectious. Particular care should be taken with young children in diapers who may have diarrhea.

How long are people contagious?

People infected with norovirus are contagious from the moment they begin feeling ill to at least 3 days after recovery. Some people may be contagious for as long as 2 weeks after recovery.Therefore, it is particularly important for people to use good hand washing and other hygienic practices after they have recently recovered from norovirus illness.

Who gets norovirus infection?

Anyone can become infected with these viruses. There are many different strains of norovirus,which makes it difficult for a person's body to develop long-lasting immunity. Therefore, norovirus illness can recur throughout a person's lifetime. In addition, because of differences in genetic factors, some people are more likely to become infected and develop more severe illness than others.

What treatment is available for people with norovirus infection?

Currently, there is no antiviral medication that works against norovirus and there is no vaccine to prevent infection. Norovirus infection cannot be treated with antibiotics. This is because antibiotics work to fight bacteria and not viruses.Norovirus illness is usually brief in healthy individuals. When people are ill with vomiting and diarrhea, they should drink plenty of fluids to prevent dehydration. Dehydration among young children, the elderly, the sick, can be common, and it is the most serious health effect that can result from norovirus infection. By drinking oral rehydration fluids (ORF), juice, or water, people can reduce their chance of becoming dehydrated. Sports drinks do not replace the nutrients and minerals lost during this illness.

Can norovirus infections be prevented?

Yes. You can decrease your chance of coming in contact with noroviruses by following these preventive steps:

• Frequently wash your hands, especially after toilet visits and changing diapers and before eating or preparing food.
• Carefully wash fruits and vegetables, and steam oysters before eating them.
• Thoroughly clean and disinfect contaminated surfaces immediately after an episode of illness by using a bleach-based household cleaner.
• Immediately remove and wash clothing or linens that may be contaminated with virus after an episode of illness (use hot water and soap).
• Flush or discard any vomitus and/or stool in the toilet and make sure that the surrounding area is kept clean.

Persons who are infected with norovirus should not prepare food while they have symptoms and for 3 days after they recover from their illness . Food that may have been contaminated by an ill person should be disposed of properly.

Influenza

Influenza, or "the flu", is a contagious respiratory illness caused by an influenza virus (type A or B).  The illness is characterized by a variety of symptoms that can include fever, headache, sore throat, nasal congestion, dry cough, fatigue and muscle aches.  Some people may also have associated gastrointestinal symptoms such as nausea, vomiting and diarrhea.  Young children, the elderly and people with certain underlying medical conditions (such as asthma, chronic lung disease, diabetes and heart disease) are particularly susceptible to influenza and its complications, most notably pneumonia and respiratory failure.  Each year, 5-15 % of the world population comes down with the flu.  3-5 million people develop severe flu-related illness and 250-500 thousand deaths occur worldwide from influenza-related complications.

Influenza viruses are spread via respiratory droplets from coughing and sneezing.  Direct person-to-person contact is the major mode of transmission for the flu, but indirect contact via the touching of contaminated environmental surfaces (and then touching your nose, mouth or eyes) can also lead to influenza infection.  After becoming infected by the virus, most people will show symptoms within 2-5 days.  The treatment for most people with the flu is rest, fluids and acetaminophen (Tylenol) or ibuprofen (Advil/Motrin) for the fever, headache and body aches.  The acute illness typically lasts for less than a week.  People infected with the influenza virus can be contagious from the day prior to the onset of symptoms up to 5 days after their illness begins.

The primary method of flu prevention is annual immunization against the particular strains of influenza that are predicted to be prevalent that year.  The most widely used method for immunization is the "flu shot".  This is an inactivated (killed virus) vaccine that is developed based upon input from worldwide medical authorities to address the influenza (types A and B) threat for a particular flu season.  It can be used in people older than 6 months of age, both those who are healthy and those with chronic medical conditions.  An alternative to the injectable vaccine is a nasal spray vaccine that is made with live, weakened viruses (LAIV or Live Attenuated Influenza Vaccine) that do not cause the flu.  It can be used in healthy people from age 5 to 49 years who are not pregnant.

The flu season in the United States and much of the Northern Hemisphere is October to May with the peak incidence for influenza running from December through March.  It takes about 2 weeks after flu vaccination for your immune system to develop adequate protection against the influenza virus.  Therefore, it is recommended that people get vaccinated in October or November near the beginning of the flu season, though later vaccination can also help to avoid getting the flu as the season progresses. 

It is important for the international traveler to realize that the flu season in the Southern Hemisphere runs from April to September (the Fall and Winter months) and that the influenza viruses for the season (as well as those used in the formation of the flu vaccine) may differ from those in the Northern Hemisphere.

In addition to an annual influenza vaccination, there are several other measures that people can utilize to lessen the likelihood of coming down with the flu.  In the event of a local or widespread flu outbreak, try to avoid contact with individuals who are sick as well as large crowds.  Cleaning your hands frequently throughout the day with soap and water or an alcohol-based hand sanitizer and being careful not to touch your eyes, nose and mouth will help to limit the chances of picking up the influenza virus from a contaminated surface.  Disinfection of contact surfaces (such as countertops, tables, telephones, door handles, faucets) in the home, schools, the workplace and other public areas can decrease the transmission of the virus.  Should you develop symptoms of the flu, it's important to minimize the chances of spreading the virus to others by staying home while ill, avoiding close contact with others and remembering to cover your mouth and nose while coughing and sneezing.

Antiviral drugs are available for both prevention and treatment of the flu.  Amantadine, rimantadine and oseltamivir (Tamiflu) are approved for prevention of the flu and can be taken for 2 weeks or more during influenza outbreaks.  Amantadine, rimantadine, oseltamivir and zanamivir (Relenza) may be prescribed to people who are diagnosed with influenza.  The medications should be started within 2 days of becoming ill and then continued for 3-5 days.  They can reduce flu symptoms, shorten the length of your illness and help to make you less contagious to others.  Oseltamivir and zanamivir are effective against both influenza type A and type B.  Amantadine and rimantadine are effective only against influenza A.