HCA Virginia - January 18, 2019
A virus attacking a cell

A virus attacking a cell

What is and isn’t really the flu

Influenza and norovirus are well known, but different, viral infections. Influenza, also called the flu, is a viral infection that affects the respiratory system. It causes mild to severe illness, and can lead to death in the most serious cases. The Centers for Disease Control and Prevention (CDC) estimates that influenza has caused between 9.3 million and 49 million illnesses every year in the United States since 2010.

A norovirus infection, on the other hand, is a food-borne illness caused by a group of viruses that attack the stomach and intestines. While norovirus is sometimes called a stomach bug or the stomach flu, it is unrelated to influenza. Both influenza and norovirus infections are contagious and can spread rapidly within a population.

Causes of Influenza and Norovirus and How They Spread


In countries with moderate temperatures, the influenza virus spreads around the world each year. Strains of the virus usually differ from one year to the next. While less likely, it is possible to get the flu when it is not flu season, especially in tropical climates.

The 2 main kinds of influenza viruses are Type A and Type B. Different strains of Type A and Type B are responsible for seasonal influenza epidemics.

There also is a Type C, but it is less common and not as virulent. Someone infected with the influenza virus may sneeze or cough, releasing droplets into the air. If you breathe in infected droplets or touch a contaminated surface, you also can become infected. The virus is transferred from your hand when you touch your mouth or nose.


Norovirus is the leading cause of foodborne illness, though other types of germs or chemicals also cause foodborne illness. Noroviruses are spread by:

A diagram of the human digestive tract

A diagram of the human digestive tract

  • Water that is not clean, such as lakes, swimming pools, wells, and water stored on cruise ships
  • Raw or poorly steamed shellfish, like clams and oysters
  • Food and drinks made by food handlers with the infection, who do not wash their hands well after using the bathroom
  • Surfaces, such as door knobs
  • Being around someone who is sick


While the flu and norovirus may share some gastrointestinal symptoms, like nausea and diarrhea, other symptoms differ:

Influenza symptoms

If you have the flu, you might infect others 1 day before symptoms start and up to 5 days (sometimes more) after you become sick. This means that you can infect others even before you know you are sick. The flu generally lasts 7 to 10 days, though a cough or fatigue may last longer.

Flu symptoms usually start abruptly and may include:

  • High fever and chills
  • Severe muscle aches
  • Severe fatigue
  • Headache
  • Decreased appetite or other gastrointestinal symptoms like nausea, vomiting, and diarrhea (more common in children than adults)
  • Runny nose, nasal congestion
  • Sneezing
  • Watery eyes or conjunctivitis
  • Sore throat
  • Cough
  • Swollen lymph nodes in the neck

Norovirus symptoms

Those infected with norovirus may experience:

  • Nausea
  • Vomiting
  • Diarrhea
  • Belly pain
  • Fever

These signs often appear within 24 to 48 hours of getting the virus, and usually last 2 to 3 days.

Risk Factors

A risk factor is something that increases your chance of developing a certain disease or condition.

Influenza risk factors

Factors that increase your chance of getting the flu include:

  • Living or working in crowded conditions, such as nursing homes, schools, the military, and daycare centers
  • Being physically or mentally disabled—people with disabilities may not be able to easily communicate their symptoms or may have trouble practicing preventive measures against the flu, putting them more at risk.

Certain groups of people are at a higher risk of developing complications from the flu. These include:

  • Children younger than 5 years old
  • Adults 65 years and older
  • Being American Indian or Native Alaskan
  • Certain health conditions, such as asthma, diabetes, or heart disease
  • A suppressed immune system
  • Pregnancy during the flu season
  • Adolescents younger than 18 who receive long-term aspirin therapy—may be at risk for Reyes syndrome
  • Living in nursing homes or other long-term care facilities
  • Obesity

Norovirus risk factors

Any person who ingests the virus is at risk of getting the infection. It is more common in young children, older adults, and crowded settings like:

  • Cruise ships
  • Restaurants
  • Nursing homes
  • Hospitals
  • Child care centers
  • Hotels


You will be asked about your symptoms and medical history, and will be given a physical exam. Often, symptoms are enough for the doctor to tell if you have a viral infection like influenza or norovirus. In some cases, your doctor may take samples from your nose or throat to confirm an influenza diagnosis.

If your doctor suspects norovirus, it usually is not necessary to find out exactly which strain of the virus is causing your infection. Stool tests may be done if your doctor wants to find out.

Treatment Options

Influenza treatment options

During recovery from the flu, it is important to rest as much as possible and to drink plenty of fluids, including water, juice, or caffeine-free tea. Medications also may be used to treat influenza, including:

Antiviral Medications

Most people with the flu do not need antiviral medication, but your doctor may prescribe it if you are in a high-risk group or if you have a severe illness. Antiviral medications generally help relieve symptoms and shorten the time you are sick. They must be taken within 48 hours of the first symptoms. Some strains of the seasonal influenza virus are resistant to these medications.

Over-the-counter (OTC) or Prescription Medications

OTC or prescription medications that may ease flu symptoms include:

  • Pain relievers and fever reducers for adults, like acetaminophen or ibuprofen
  • Cough remedies—to make a cough more productive or to suppress a cough
  • Decongestants—Do not use nasal sprays longer than 3 to 5 days. If so, you may experience an increase in congestion when you stop using the spray, known as a rebound effect.
  • Antihistamines

Note: OTC cough and cold products should not be used to treat infants or children less than 2 years old. Rare, but serious side effects have been reported. They include death, convulsions, rapid heart rates, and decreased levels of consciousness. Serious side effects also have been reported in children aged 2 to 11 years. Research is ongoing to test the safety of OTC products for this age group.

Note: Aspirin is not recommended for children with a current or recent viral infection. Check with your doctor before giving your child aspirin.

Norovirus treatment options

A norovirus infection often is brief and mild, and will go away on its own. Most people will get better by resting and drinking plenty of fluids. Rarely, you may need intravenous (IV) fluids if you have severe dehydration. Antibiotics are not helpful in treating viruses. There are no antiviral medicines or vaccines used to fight or prevent a norovirus infection.


Influenza Prevention

The best way to prevent getting the flu is to be vaccinated every year, since the virus may change every season. Vaccinations come in the form of an injection (shot) or a nasal spray.

The flu shot is approved for all people aged 6 months and older. Note: Children 8 years and younger may need 2 shots, 4 weeks apart.

The nasal spray is approved for non-pregnant individuals between the ages of 2 and 49, who do not have a serious underlying medical condition.

For the best protection, get vaccinated as soon as the vaccine is available in your area. Vaccinations are offered throughout the flu season, which may begin as early as October. It takes about 2 weeks for the vaccine to protect you against the flu.

People who should not receive flu vaccine

There are people who should not be vaccinated, including:

  • Children less than 6 months old
  • Those who had a severe reaction to vaccination in the past
  • Those with a history of Guillain-Barre syndrome
  • Those who are very sick and have a fever

General Measures to Reduce Your Flu Risk

There are general measures you can take to reduce your risk of getting the flu:

  • Wash hands frequently, especially when you come in contact with someone who is sick. Wash your hands for 15-20 seconds with soap and water. Rubbing alcohol-based cleaners on your hands also is helpful.
  • Avoid close contact with people who have respiratory infections. If you have to be in close contact with a sick person, wear a facemask or disposable respirator.
  • Cover your mouth and nose with a tissue when you cough or sneeze. Throw away the tissue after you use it. Coughing or sneezing into your elbow or upper sleeve also helps.
  • Do not share drinks or personal items.
  • Do not bite your nails or put your hands near your eyes, mouth, or nose.
  • Keep surfaces clean by wiping them with a household disinfectant.
  • Use the hot setting on your washing machine when washing infected laundry.

Norovirus Prevention

To help lower your chance of getting a norovirus infection:

  • Wash your hands well with soap and water:
    • After using the bathroom
    • After changing diapers
    • Before you make or eat food
  • If you are caring for someone who is infected, make sure that the person washes his or her hands often.
  • If you are ill, or caring for someone who is ill, clean contaminated surfaces right away using a disinfectant cleaner. Remove and wash soiled linens with hot water and soap.
  • Wash fruits and vegetables.
  • Cook oysters and clams before eating them.
  • Throw away contaminated food.


Centers for Disease Control and Prevention (CDC)

National Institute of Allergies and Infectious Diseases

World Health Organization (WHO)

April 15, 2021
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