INTRODUCTION

Influenza (commonly called the flu) is a highly contagious illness that can occur in children or adults of any age. It occurs more often in the winter months because people spend more time in close contact with one another. The flu is spread easily from person-to-person by coughing, sneezing, or touching surfaces.

Every year, complications of the flu require more than 200,000 people in the United States to be hospitalized. Serious illness is more likely in the very young, older adults, pregnant women, and people who have certain health problems such as asthma or other forms of lung disease.

There have been several widespread flu outbreaks (called pandemics), which led to the deaths of many of people worldwide. These outbreaks occurred when influenza viruses formed (often from pigs or birds) and humans became infected because they had no immunity to these viruses.

This article discusses the symptoms and treatment of seasonal, swine H1N1, and avian flu. Treatments to prevent the flu, including the flu shot, are discussed separately. (See "Patient information: Influenza prevention".)

SWINE H1N1 FLU

Swine influenza is a type of flu virus that infects pigs. A new strain of H1N1 influenza, which contains parts of swine, avian, and human influenza viruses, was first seen in March 2009 in Mexico. Human cases with this type of virus have now been seen around the world. (See "Epidemiology of pandemic H1N1 influenza ('swine influenza')".)

Symptoms of infection with the swine H1N1 flu virus are similar to those of seasonal flu, including:

• Fever (temperature higher than 100ºF or 37.8ºC) and chills
• Cough
• Sore throat
• Body aches or headache
• Fatigue
• Vomiting and diarrhea (not typical with seasonal flu)

The virus can be spread by coughing or sneezing, or by touching surfaces contaminated with the virus and then touching your nose or mouth. It is not possible to become infected with swine H1N1 influenza by eating pork.

You can spread the infection starting one day before you have symptoms until at least 24 hours after your fever has gone (without taking anti-fever medicines). Children and people with a weakened immune system may be contagious for more than seven days.

You can reduce the chance of becoming sick with this strain of influenza virus by following simple infection control measures. (See "Patient information: Influenza prevention", section on 'Infection control'.)

An antiviral medicine, such as oseltamivir (Tamiflu) or zanamivir (Relenza) can also help to prevent infection with swine H1N1 flu. This treatment may be recommended if you have been in recent, close contact with someone who is infected with swine H1N1 flu. Treatments to prevent the flu are discussed separately. (See "Patient information: Influenza prevention" and "Antiviral prophylaxis for the prevention of pandemic H1N1 influenza ('swine influenza')".)

Antiviral medicines can also be used to treat people with swine H1N1 influenza. (See 'Antiviral treatment' below and "Treatment of pandemic H1N1 influenza ('swine influenza')".)

If you think you could be infected with the new swine H1N1 flu virus, do not go to work or school to avoid spreading your illness. You do not have to see a doctor or nurse unless you have worrisome symptoms or you are at risk for severe influenza. People at risk for severe influenza include:

• Children under five years old (especially children under 2 years old)
• Adults age 65 and older
• Pregnant women
• People with certain medical problems like asthma or a weakened immune system

People in these groups should call their doctor or nurse for advice at the first sign of the flu. (See 'When to seek help' below.)

More information about the swine H1N1 flu virus is available from the United States Center for Disease Control and Prevention (www.cdc.gov/h1n1flu/).

AVIAN FLU

Avian influenza (bird flu) is caused by a strain of influenza virus that originally infected birds. Infected birds include chickens, ducks, geese, and other birds. More recently, the infection has been found in cats and dogs that were probably infected after eating birds.

There are several strains of avian flu; strain H5N1 virus is the cause of recent concern since it has led to several deaths, mostly in Asia. To date, avian flu has primarily spread from bird-to-bird, and much less commonly from bird-to-human; human-to-human transmission has occurred rarely. Most humans who became infected with avian flu had direct contact with sick or dead poultry or wild birds, or had visited a live poultry market.

Avian flu is a severe form of the flu and there is little natural resistance in the population. At least one antiviral medicine (oseltamivir) might improve the chance of surviving the infection. (See 'Antiviral treatment' below.)

There is a vaccine to prevent avian flu. The vaccine is not commercially available, but has been stockpiled by the United States government in case it is needed in the future.

Updated information about avian influenza is available from the United States Center for Disease Control and Prevention (www.cdc.gov/flu/avian/).

SEASONAL FLU SYMPTOMS

Symptoms of seasonal flu can vary from person to person, but usually include:

• Fever (temperature higher than 100ºF or 37.8ºC)
• Headache and muscle aches
• Fatigue
• Cough and sore throat may also be present

People with the flu usually have a fever for two to five days. This is different than fever caused by other upper respiratory viruses, which usually resolve after 24 to 48 hours.

Some people have cold-like symptoms (runny nose, sore throat) during the flu while others have fever and muscle aches. Flu symptoms usually improve over two to five days, although the illness may last for a week or more. Weakness and fatigue may persist for several weeks (table 1).

Flu complications — Complications of influenza occur in some people; pneumonia is the most common complication. Pneumonia is a serious infection of the lungs, and is more likely to occur in people over the age of 65, people who live in long term care facilities (nursing homes), and those with other illnesses such as diabetes or conditions affecting the heart or lungs. (See "Patient information: Pneumonia in adults".)

FLU DIAGNOSIS

Influenza is usually diagnosed based on symptoms (fever, cough and muscle aches). Lab testing for influenza is performed in certain cases, such as during a new influenza outbreak in a community.

FLU TREATMENT

When to seek help — Most people with the flu recover within one to two weeks without treatment. However, serious complications of the flu can occur. Call your doctor or nurse immediately if:

• You feel short of breath or have trouble breathing
• You have pain or pressure in your chest or stomach
• You have signs of being dehydrated, such as dizziness when standing or not passing urine
• You feel confused
• You cannot stop vomiting or you cannot drink enough fluids

In children, you should seek help if the child has any of the above or if the child:

• Has blue or purplish skin color
• Is so irritable that he or she does not want to be held
• Does not have tears when crying (in infants)
• Has a fever with a rash
• Does not wake up easily

Treat symptoms — Treating the symptoms of influenza can help you to feel better, but will not make the flu go away faster.

• Rest until the flu is fully resolved, especially if the illness has been severe
• Fluids — Drink enough fluids so that you do not become dehydrated. One way to judge if you are drinking enough is to look at the color of your urine. Normally, urine should be light yellow to nearly colorless. If you are drinking enough, you should pass urine every three to five hours.
• Acetaminophen (such as Tylenol® and other brands) can relieve fever, headache, and muscle aches. Aspirin, and medicines that include aspirin (eg, bismuth subsalicylate; PeptoBismol), are not recommended for children under 18 because aspirin can lead to a serious disease called Reye syndrome.
• Cough medicines are not usually helpful; cough usually resolves without treatment. We do not recommend cough or cold medicine for children under age six years. (See "Patient information: The common cold in children".)

Antiviral treatment — Antiviral medicines can be used to treat or prevent influenza. When used as a treatment, the medicine does not eliminate flu symptoms, although it can reduce the severity and duration of symptoms by about one day. Not every person with influenza needs an antiviral medicine; the decision is based upon your risk of developing complications of influenza.

Available antiviral medicines include oseltamivir (Tamiflu®), zanamivir (Relenza®), rimantadine (Flumadine®), and amantadine (Symmetrel®). Antiviral treatment is most effective for seasonal influenza when it is taken within the first 48 hours of flu symptoms.

The best antiviral medicine depends upon the type of influenza virus, if the virus could be resistant, and some individual factors. A doctor or nurse should make this decision. (See "Prevention of seasonal influenza in adults" and "Treatment of seasonal influenza in adults" and "Antiviral drugs for the prevention and treatment of influenza in children" and "Treatment of pandemic H1N1 influenza ('swine influenza')" and "Antiviral prophylaxis for the prevention of pandemic H1N1 influenza ('swine influenza')".)

Side effects — Zanamivir and oseltamivir can cause mild side effects, including nausea and vomiting; zanamivir, which is inhaled, can cause difficulty breathing in some cases. Most people are able to continue the medicine despite the side effects.

Antibiotics — Antibiotics are NOT useful for treating viral illnesses such as influenza. Antibiotics should only used if there is a bacterial complication of the flu such as bacterial pneumonia, ear infection, or sinusitis. Antibiotics can cause side effects and lead to development of antibiotic resistance.

Complementary and alternative treatments — There are a wide variety of herbal, homeopathic, and other complementary and alternative treatments that are marketed for influenza. Unfortunately, there have been few well designed studies to evaluate their efficacy and safety.

One homeopathic remedy, oscillococcinum, does not work to prevent influenza but may shorten influenza symptoms (by an average of eight hours, with a range from one to twelve hours) [1]. There are no known serious side effects but its safety has not been well-studied.

PREVENTING FLU

Treatments to prevent influenza are discussed separately. (See "Patient information: Influenza prevention".)

WHERE TO GET MORE INFORMATION

Your healthcare provider is the best source of information for questions and concerns related to your medical problem.

This article will be updated as needed every four months on our web site (www.uptodate.com/patients).

Related topics for patients, as well as selected articles written for healthcare professionals, are also available. Some of the most relevant are listed below.

Patient level information

Patient information: Influenza prevention
Patient information: Pneumonia in adults
Patient information: The common cold in children

Professional level information

Clinical manifestations and diagnosis of avian influenza
Clinical manifestations and diagnosis of seasonal influenza in adults
Epidemiology of influenza
Epidemiology, transmission, and pathogenesis of avian influenza
Pharmacology of antiviral drugs for influenza
Prevention of seasonal influenza in adults
Seasonal influenza vaccination in adults
Treatment and prevention of avian influenza
Treatment of seasonal influenza in adults
Epidemiology of pandemic H1N1 influenza ('swine influenza')
Antiviral prophylaxis for the prevention of pandemic H1N1 influenza ('swine influenza')
Treatment of pandemic H1N1 influenza ('swine influenza')
Antiviral drugs for the prevention and treatment of influenza in children

The following organizations also provide reliable health information.

• National Library of Medicine

      (www.nlm.nih.gov/medlineplus/flu.html)

• National Institute of Allergy and Infectious Diseases

      (www3.niaid.nih.gov/topics/Flu/)

• Centers for Disease Control and Prevention (CDC)

      (www.cdc.gov/h1n1flu/)

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