Influenza, which is commonly called flu, is a viral infection that attacks your nose, throat, and lungs. Most people recover from the flu within a few days.

Influenza Overview

Reviewed: May 19, 2014

Influenza (flu) is a contagious respiratory illness caused by influenza viruses. The viruses pass through the air and enter your body through your nose or mouth. Flu can cause mild to severe illness. Serious outcomes of flu infection can result in hospitalization or death. Some people, such as elderly people, newborns and young children, and people with certain health conditions, are at high risk for serious flu complications. Between 5% and 20% of people in the U.S. get the flu each year. The best way to prevent the flu is by getting vaccinated each year.

Symptoms of the flu come on suddenly and are worse than those of the common cold. Colds rarely cause a fever or headaches. Flu almost never causes an upset stomach. And "stomach flu" is not really flu at all, but gastroenteritis.

Most people with the flu recover on their own without medical care. People with mild cases of the flu should stay home and avoid contact with others, except to get medical care. If you get the flu, your health care provider may prescribe medicine to help your body fight the infection and lessen symptoms.

The main way to keep from getting the flu is to get a yearly flu vaccine. Good hygiene, including hand washing, can also help.

Influenza Symptoms

Symptoms of the flu come on suddenly. They may include:

  • body or muscle aches
  • chills
  • cough
  • fever
  • headache
  • sore throat
  • fatigue

Flu symptoms generally last less than a week.

Influenza Causes

Flu viruses travel through the air in droplets when someone with the infection coughs, sneezes or talks. People with flu can spread it to others up to about 6 feet away. You can inhale the droplets directly, or you can pick up the germs from an object and then transfer them to your eyes, nose or mouth.

People with the virus are likely contagious from the day or so before symptoms first appear until about 5 days after symptoms begin, though sometimes people are contagious for as long as 10 days after symptoms appear. Children and people with weakened immune systems may be contagious for a slightly longer time.

Influenza viruses are constantly changing, with new strains appearing regularly. If you have had influenza in the past, your body has already made antibodies to fight that particular strain of the virus. If future influenza viruses are similar to those you have encountered before, either by having the disease or by vaccination, those antibodies may prevent infection or lessen its severity. Antibodies against flu viruses you have encountered in the past cannot protect you from new influenza subtypes that can be very different immunologically from what you had before.

Factors that may increase your risk of developing influenza or its complications include:

  • Age. Flu tends to occur more frequently in young children and older adults.
  • Living conditions. People who live in facilities along with many other residents, such as nursing homes or military barracks, are more likely to develop the flu.
  • Weakened immune system. Cancer treatments, anti-rejection drugs, corticosteroids and HIV/AIDS can weaken your immune system, which makes it easier for you to catch influenza and may also increase your risk of developing complications.
  • Chronic illnesses. Chronic conditions, such as asthma, diabetes or heart problems, may increase your risk of influenza complications.
  • Pregnancy. Pregnant women are more likely to develop influenza complications, particularly in the second and third trimesters.
  • Obesity. People who are extremely overweight or obese have an increased risk of complications from flu.

Influenza Diagnosis

A respiratory illness might be the flu if it is accompanied by fever, cough, sore throat, runny or stuffy nose, body aches, headache, chills and fatigue. Some people may have vomiting and diarrhea. People may or may not have a fever with the flu. Flu viruses usually cause the most illness during the colder months of the year, but influenza can also occur outside of the typical flu season. Additionally, other viruses can also cause respiratory illness similar to the flu. Therefore, it is impossible to tell for sure if you have the flu based on symptoms alone.

Your health care provider may diagnose you with flu based on your symptoms and their clinical judgment or they may choose to use an influenza diagnostic test. During an outbreak of respiratory illness, testing for flu can help determine if flu viruses are the cause of the outbreak. Flu testing can also be helpful for some people with suspected flu who are pregnant or have a weakened immune system, and for whom a diagnosis of flu can help their doctor make decisions about their care.

Living With Influenza

The Centers for Disease Control and Prevention recommends annual flu vaccination for everyone over the age of 6 months. Each year's seasonal flu vaccine contains protection from the influenza viruses that are expected to be the most common during that year's flu season. The vaccine is typically available as an injection or as a nasal spray.

If you develop the flu, these measures may help ease your symptoms:

  • Drink plenty of liquids. Choose water, juice, and warm soups to prevent dehydration.
  • Rest. Get more sleep to help your immune system fight infection.
  • Consider pain relievers. Use an over-the-counter pain reliever, such as acetaminophen (Tylenol, others) or ibuprofen (Advil, Motrin IB, others), to relieve the achiness associated with influenza.

To avoid spreading the influenza virus, people should stay away from sick people and stay home if they are sick. It also is important to wash your hands often with soap and water. If soap and water are not available, use an alcohol-based hand sanitizer. Linens, eating utensils, and dishes belonging to those who are sick should not be shared without first washing thoroughly. Frequently touched surfaces should be cleaned and disinfected at home, work and school, especially if someone is ill. Finally, contain your coughs and sneezes. Cover your mouth and nose when you sneeze or cough. To avoid contaminating your hands, cough or sneeze into a tissue or into the inner crook of your elbow.

Influenza Treatments

The goal of treatment of flu is relief of symptoms and prevention of complication associated with the infection.

Usually, bed rest and plenty of fluids is sufficient treatment for the flu. Over-the-counter pain relievers, such as acetaminophen (Tylenol, others) or ibuprofen (Advil, Motrin IB, others), can be used to relieve the achiness associated with influenza.

In some cases, your doctor may prescribe an antiviral medication, such as oseltamivir (Tamiflu) or zanamivir (Relenza). If taken soon after you notice symptoms, these drugs may shorten your illness by a day or so and help prevent serious complications.

Influenza Other Treatments

Influenza Prognosis