INFLUENZA OVERVIEW — Influenza (commonly called the flu) is a highly contagious viral illness that can occur in children or adults of any age. It occurs most frequently in the winter months as individuals spend more time in close contact with one another. The virus is spread easily from person-to-person via infected respiratory secretions.

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

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

SWINE H1N1 FLU — Swine influenza is a type of flu virus that infects pigs. A new strain of H1N1 influenza, which contains parts of the swine, avian, and human influenza virus, was first seen in March 2009 in Mexico. Cases of human infection with this type of virus have been seen around the world. (See "Epidemiology, clinical manifestations, and diagnosis of pandemic H1N1 influenza ('swine influenza')").

Symptoms of infection with the swine H1N1 flu virus are similar to those of seasonal flu, including fever, cough, sore throat, body aches, headache, chills, and fatigue; vomiting and diarrhea have also been common, which differs from seasonal influenza. 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 are considered contagious beginning one day before symptoms appear until at least 24 hours after your fever (100ºF or 37.8ºC) has resolved (without taking anti-fever medications). 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, discussed below. (See "Infection control" below).

An antiviral medication, such as oseltamivir or zanamivir, 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 with a suspected or confirmed case of swine H1N1 flu. A discussion of preventive treatment with antiviral medications is available separately. (See "Patient information: Influenza prevention" and see "Prevention of pandemic H1N1 influenza ('swine influenza')").

Antiviral medications, such as oseltamivir or zanamivir, can also be used to treat people with swine H1N1 influenza to reduce the severity and duration of symptoms. (See "Antiviral drugs" below and see "Treatment of H1N1 influenza A (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 healthcare provider unless you develop worrisome symptoms. In this case, you should call your healthcare provider. (See "When to seek help" below).

More information about preventing the spread of the new 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 migratory 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 resulted in 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.

A major concern with avian flu is that it is a severe form of the flu and there is little natural resistance in the population. At least one antiviral medication (oseltamivir) may improve the chance of surviving infection. (See "Antiviral drugs" below).

A vaccine has been developed 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 a fever, headache, muscle aches, and fatigue; cough and sore throat may also be present. People with the flu usually have fevers for two to five days, in contrast to fevers caused by other upper respiratory viruses that usually resolve after 24 to 48 hours.

Some people experience cold-like symptoms 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 (show figure 1).

Flu complications — Complications of influenza occur in some people; pneumonia is the most common. Pneumonia is a serious infection of the lungs, and is more likely to occur in people over the age of 65, residents of long term care facilities, and those with underlying 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 the typical symptoms (fever, cough and muscle aches). Laboratory testing for influenza is performed in selected 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. If you develop any of the following, contact your healthcare provider immediately.

• Shortness of breath or difficulty breathing
• Purple or blue discoloration of the lips
• Pain or pressure in the chest or abdomen
• Signs of dehydration, such as dizziness when standing, not passing urine, or, in infants, lack of tears when crying
• Confusion or less responsiveness than usual
• Seizures or convulsions
• Severe vomiting or unable to keep fluids down

Treat symptoms — Treating the symptoms of influenza can help you to feel better, but will not shorten the duration of the flu.

• 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.

• Acetaminophen (such as Tylenol® and other brands) to relieve fever, headache, and muscle aches. Aspirin, and medicines that include aspirin (eg, bismuth subsalicylate; PeptoBismol), should be avoided, particularly in children under 18, because it can lead to a serious disease called Reye syndrome.

• Cough suppressants 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 drugs—- Antiviral drugs may be recommended to treat or prevent influenza. When used as a treatment, the drug does not eliminate flu symptoms, although it can reduce the severity and duration of symptoms by two to three days. Not every person with influenza requires treatment with an antiviral medication; the decision is based upon your risk of developing complications of influenza.

Available antiviral medications 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 medication for the treatment or prevention of influenza depends upon the type of influenza virus, the likelihood of drug resistance, and some patient-related factors. This decision should be made by a healthcare provider. (See "Antiviral drugs for the prevention of influenza in adults" and see "Antiviral drugs for the treatment of influenza in adults" and see "Antiviral drugs for the prevention and treatment of influenza in children" and see "Treatment of H1N1 influenza A (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 patients are able to continue the medication 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, appears to be ineffective for prevention of influenza but may have a very modest benefit in reducing the duration of influenza symptoms (by an average of eight hours, with a range from one to twelve hours) [1]. Serious side-effects have not been reported but its safety has not been evaluated extensively.

PREVENTING FLU — One of the most effective ways to prevent infection with influenza is the influenza vaccine, which is given once per year, usually in the fall. The flu vaccine is discussed in a separate topic. (See "Patient information: Influenza prevention").

Antiviral medications — Antiviral medications are sometimes given to people who have been exposed to influenza but are not yet ill. This treatment is discussed separately. (See "Patient information: Influenza prevention").

Infection control — For people with the flu, it is important to use simple infection control measures to prevent the spread of infection. Infection control measures can help to prevent the spread of influenza.

Seasonal flu prevention

• Frequent handwashing with soap and water can also help limit the spread of influenza. Alcohol-based hand sanitizers can be used when soap and water are not available. Whether you are infected with the flu or are caring for someone with the flu, you should wash your hands frequently.

• Because influenza is spread by contact with infected respiratory secretions, people with influenza should avoid face-to-face contact with anyone while sick.

• Cover the mouth and nose should while coughing or sneezing, and dispose of tissues immediately. Sneezing/coughing into the sleeve of one's clothing (at the inner elbow) is another means of containing sprays of saliva and secretions and has the advantage of not contaminating the hands.

Swine H1N1 flu prevention — In addition to the precautions listed above, further precautions are recommended to prevent the spread of swine H1N1 flu virus. (See "Swine H1N1 Flu" above).

• People who are infected with swine flu should stay home, away from other adults and children, for at least 24 hours after their fever (100ºF or 37.8ºC) has resolved (without taking anti-fever medications).

• If face-to-face contact is necessary, the infected person should wear a face mask.

• There are numerous options for face masks. N95 respirators are recommended for anyone caring for an infected person. An N95 respirator fits snugly on your face and can filter out small particles that can be inhaled around the edges of other types of face masks. However, it is harder to breathe through an N95 mask for long periods of time compared to a looser fitting mask.

• If possible, the infected person should stay in a room separate from common areas of the house, keep the door closed, and use a separate bathroom while ill. The bathroom should be cleaned daily with a disinfectant.

• Pregnant women should avoid caring for adults or children infected with the swine H1N1 flu.

SUMMARY

• Influenza (commonly called the flu) is a highly contagious viral illness that can occur in children or adults of any age.

• Influenza usually begins with fever, headache, muscle aches, and fatigue; cough and sore throat may also be present. Symptoms usually improve over two to five days, although the illness may last for a week or more. Weakness and fatigue may continue for several weeks.

• Influenza is usually diagnosed based on the typical symptoms that occur during an outbreak in the community. The diagnosis can be confirmed with testing, although this is not always necessary.

• You can relieve the symptoms of influenza by getting plenty of rest, drinking adequate fluids, and taking medication to treat fever and body aches, such as acetaminophen (Tylenol® and other brands).

• Antiviral prescription medications may be recommended in some cases to treat or prevent influenza. They are best taken as early as possible after the onset of flu symptoms, ideally within the first 48 hours.

• Influenza can be prevented by the influenza vaccine and simple infection control measures such as hand washing. Antiviral medications may help prevent infection after exposure.

• Influenza is spread by contact with infected respiratory secretions (from coughing, sneezing, or phlegm). People infected with the flu should limit face-to-face contact with uninfected family and friends. Hand washing with soap and water or alcohol-based hand rubs can reduce spread. The mouth and nose should be covered while coughing or sneezing, and tissues should be disposed of immediately.

WHERE TO GET MORE INFORMATION — Your healthcare provider is the best source of information for questions and concerns related to your medical problem. Because no two people are exactly alike and recommendations can vary from one person to another, it is important to seek guidance from a provider who is familiar with your individual situation.

This discussion will be updated as needed every four months on our web site (www.uptodate.com/patients). Additional topics as well as selected discussions written for healthcare professionals are also available for those who would like more detailed information.

Some of the most pertinent include:

Patient Level Information:
 

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

Professional Level Information:

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

A number of web sites have information about medical problems and treatments, although it can be difficult to know which sites are reputable. Information provided by the National Institutes of Health, national medical societies and some other well-established organizations are often reliable sources of information, although the frequency with which they are updated is variable.

• National Library of Medicine

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

• National Institute of Allergy and Infectious Diseases

      Bethesda, MD 20892-2520
      (www.niaid.nih.gov/)
 

• Centers for Disease Control and Prevention (CDC)

      Toll-free: (800) 311-3435
      (www.cdc.gov/epo/mmwr/mmwr.html)
 

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