Influenza, or "flu," is caused by a virus infecting the respiratory system, meaning your nose, throat, bronchial tubes, and lungs. Flu symptoms are usually more severe than those of the common cold and are more likely to affect other parts of your body. Flu also tends to come on suddenly while colds can develop gradually.

Flu is very contagious, spreading easily from one person to the next. Most people with healthy immune systems will get over the flu within 2 weeks, but young children, older adults, and people with chronic illnesses are more likely to develop complications such as pneumonia. About 35,000 people die of flu each year in the United States.

There are three types of flu viruses: A, B, and C. Type A viruses are the ones responsible for worldwide epidemics, such as the one in 1918 that killed as many as 50 million people worldwide. The avian or bird flu is a type A flu virus.

The best way to protect yourself from the flu is to get an annual vaccine (flu shot).

• Fever that comes on suddenly (usually above 101°F [38.3°C])
• Chills and sweats
• Headache
• Muscle or body aches
• Fatigue
• Dry cough
• Sore throat
• Sneezing, runny nose, stuffy nose
• Loss of appetite
• Nausea, vomiting, or diarrhea, especially in children
• Weakness and confusion, especially among the elderly

Influenza is caused by viruses that are spread through the air by sneezes and coughs, or by touching a surface a person with the flu has touched and transmitting the virus to your mouth or nose. Some flu viruses cause a very mild illness, or none at all. Others cause serious, widespread illness.

Since there are many types of influenza virus, and because they change over time, scientists develop a new flu vaccine every fall. Getting vaccinated before the flu season starts reduces your chances of getting the flu and helps you recover faster if you do get it. You should not take the vaccine if you have a severe allergy to eggs because the viruses for the vaccines are grown in chick embryos. See Risk Factors for list of people who should get the vaccine every year.

Infants and young children, as well as senior adults, are considered at highest risk of complications from flu. Other risks include:

• Having a chronic illness, such as heart disease
• Having a weakened immune system, from medications or HIV
• Pregnant women
• Working in health care
• Working in childcare
• Living in a nursing home

If you are at risk for complications, you should get an annual flu shot (see Preventive Care).

Your doctor will probably be able to diagnose flu from a physical exam and a description of your symptoms. Your doctor may take a chest x-ray if there is concern about complications such as pneumonia.

The best way to prevent the flu is by getting a flu shot. Annual flu shots are recommended if you:

• Are 50 or older
• Have chronic heart, lung, or kidney disease
• Live in an institution (such as a nursing home)
• Have a weakened immune system
• Have sickle cell anemia

You should not receive the vaccine if you are severely allergic to eggs.

You can also cut your risk of flu by washing your hands frequently during flu season. In one study, using alcohol-based sanitizer, along with good respiratory hygiene, reduced school absences by 26% and laboratory confirmed influenza A infections by 52%.

Bed rest and drinking plenty of fluids are usually enough to treat flu. Mild over-the-counter pain relievers and fever reducers (such as acetaminophen, or Tylenol, and ibuprofen, or Advil), can help relieve fever and muscle aches. If you are at high risk for complications (see Risk Factors), then your doctor may prescribe antiviral medications, drugs that fight the virus. They must be started within 2 days to be effective. Studies show that taking antiviral medications within 2 days of the onset of symptoms may reduce the duration of the illness by a day. Certain herbs, supplements, and homeopathic remedies may help relieve symptoms.

Lifestyle

• Drink lots of fluids.
• Rest to restore your energy and avoid complications like pneumonia.
• Eat a diet rich in fresh fruits and vegetables. These foods provide lots of antioxidants, substances that may help boost your immune system, especially vitamins A and C.
• Regularly exercising may cut your risk of flu and help your body respond better to a flu shot.
• Reduce stress and your reaction to stress. Consider yoga, tai chi, or other forms of relaxation on an ongoing basis. Stress can put you at increased risk for viruses like influenza.

Medications

• Pain and fever reducers, include ibuprofen (Advil, Motrin) and acetaminophen (Tylenol) for fever reduction and relief of minor aches and pain. Children under 19 should not take aspirin due to the risk of a rare but serious illness called Reye syndrome.
• Decongestants. Help open your nasal passages so you can breathe easier. If decongestant nasal sprays or drops are used for more than 3 days, however, they can cause rebound congestion. Decongestants are often combined in cold and flu medicines with antihistamines, cough suppressants, and pain relievers. People with heart disease, high blood pressure, diabetes, or glaucoma should not take decongestants. Popular brands of decongestants include Sudafed, Afrin, and Neo-Synephrine.
• Cough medicines. Cough suppressants (for a dry cough) or expectorants (for a wet, productive cough that brings up mucus) are available over the counter and by prescription.

Antiviral medications. Several antiviral medications have been approved by the Food and Drug Administration (FDA) to treat flu. However, a number of flu viruses have developed resistance to some of the medications. In addition, using these medications may help contribute to other strains of flu becoming resistant. These drugs must be started within 48 hours of becoming sick to be effective. Medications include:

• Oseltamivir (Tamiflu): The FDA requires Tamiflu to carry a warning that people who have the flu, especially children, may be at increased risk of confusion and injuring themselves after taking Tamiflu.
• Zanamivir (Relenza)
• Amantadine (Symmetrel): Recently, most flu viruses in the U.S. have been resistant to Symmetrel
• Rimantadine (Flumadine): Recently, most flu viruses in the U.S. have been resistant to Flumadine

Nutrition and Dietary Supplements

Because supplements may have side effects or interact with medications, you should take them only under the supervision of a knowledgeable health care provider. Be sure to talk to your doctor about any supplements you are taking or considering taking.

• Warm liquids. Chicken soup and warm liquids, such as broth or tea, can help soothe a sore throat and loosen mucus, which in turn helps ease congestion from the flu.
• Probiotics (Lactobacillus). So-called "good" bacteria or probiotics help prevent infections in the intestines, and there is preliminary evidence that they might help prevent colds, too, although they have not been studied for the flu. One study found that children in daycare centers who drank milk fortified with Lactobacillus had fewer and less severe colds. Another study of children in daycare found those who took a specific combination of Lactobacillus and Bifidobacterium had fewer flu-like symptoms. Several studies that examined probiotics combined with vitamins and minerals also found a reduction in the number of colds caught by adults, although it is not possible to say whether the vitamins, minerals, or probiotics were most responsible for the benefit. People with weakened immune systems, or who take drugs to suppress the immune system, should not take probiotics except under a doctor's supervision.
• Zinc. Your body needs zinc for proper immune function, so it has long been thought that zinc could help protect against catching a cold or flu. But the evidence has been mixed, with some studies finding a benefit from zinc lozenges and others showing no effect. A review of studies that compared zinc to placebo found that most of them had flaws that made any positive results unreliable. Only 4 studies were deemed reliable, and 3 found no benefit from zinc lozenges. The remaining positive study suggested that zinc nasal sprays might help reduce nasal stuffiness, but zinc nasal sprays have been reported to cause permanent loss of smell, and are not recommended. If you decide to try zinc lozenges for a cold, remember that getting too much zinc (more than 50 mg per day over a long period of time) can be dangerous. People taking cisplatin, penicillamine, and quinolone, or tetracycline antibiotics should not take zinc.
• Vitamin D. In one randomized, double-blind, placebo-controlled study, school-aged children who took 1200 IU of vitamin D daily had fewer cases of flu than those who took placebo. Because of how vitamin D works with calcium in the body, people with the following conditions should ask their doctor before taking vitamin D: kidney disease, sarcoidosis, lymphoma, atherosclerosis, hyperparathyroidism, and tuberculosis. Vitamin D may interact with these medications: atorvastatin (Lipitor), calcipotriene (Dovonex), digoxin (Lanoxin), diltiazem (Cardizem, Dilacor), thiazide diuretics (water pills), and verapamil (Calan, Verelan), among others. If you take any medication, ask your doctor before taking vitamin D.

Herbs

The use of herbs is a time-honored approach to strengthening the body and treating disease. Herbs, however, can trigger side effects and can interact with other herbs, supplements, or medications. For these reasons, you should take herbs with care, under the supervision of a health care practitioner. Before giving any herbs to a child to treat the flu, talk to your pediatrician.

• Andrographis (Andrographis paniculata). One study found that Andrographis, an herb used in Ayurvedic medicine, combined with eleuthero or Siberian ginseng (Eleutherococcus senticosus) in a formula called Kan Jang, helped reduce symptoms and shorten the duration of the flu. More research is needed. Pregnant or breastfeeding women should not take Andrographis. People with diabetes, or those who take blood thinners or blood pressure medications, should not take Andrographis without first asking their doctor.
• Echinacea (Echinacea purpurea). Although evidence on whether echinacea works to prevent or treat colds and flu has gone back and forth, recent evidence is encouraging. An analysis of 14 scientific studies found that people who took Echinacea reduced their risk of getting a cold by 58% and reduced the duration of a cold by an average of a day and a half. However, many of the studies used echinacea in combination with another herb or vitamin, so it is impossible to say which one was responsible for the benefit. The analysis looked at colds, not flu, so the evidence may be more mixed as to whether echinacea can help prevent flu. Echinacea should not be used by women who are pregnant or breastfeeding, or by anyone taking drugs that suppress the immune system (such as corticosteroids or methotrexate). People who are allergic to ragweed should ask their health care provider before taking echinacea.
• Garlic (Allium sativum). Some studies suggest taking garlic may help reduce your risk of getting an upper respiratory infection, such as a cold or flu. More studies are needed to tell whether garlic has any true benefit for flu, however. Because garlic can increase the risk of bleeding, people who take blood thinners, such as warfarin (Coumadin), clopidogrel (Plavix), or aspirin should not take garlic. Women who are pregnant or breastfeeding should talk to their doctor before taking garlic supplements. Garlic can potentially interfere with a variety of medications, including some medications used in the treatment of HIV/AIDS and birth control medication, among others. Talk to your physician if you are taking any medications.
• Elder or elderberry (Sambucus nigra). By lessening congestion and possibly increasing perspiration, elder may help reduce the symptoms of colds and flu. One study suggested that using a standardized elderberry extract, Sambucus, could shorten the duration of flu by about 3 days. Sambucus also contains other herbs plus vitamin C, so it is not known whether elder by itself would have the same effect. Pregnant and breastfeeding women should not take elder. People with diabetes or high blood pressure, or those who take blood-thinners, such as aspirin or warfarin (Coumadin), should ask their doctor before taking elder.
• Eucalyptus (Eucalyptus globulus). Eucalyptus is used in many remedies to treat cold and flu symptoms, particularly cough. It can be found in many lozenges, cough syrups, and vapor baths throughout the United States and Europe. Fresh leaves can be used in teas and gargles to soothe sore throats. Ointments containing eucalyptus leaves are also applied to the nose and chest to relieve congestion and loosen phlegm. DO NOT take eucalyptus oil by mouth because it can be poisonous.
• Goldenseal (Hydrastis canadensis). Goldenseal is often combined with echinacea in herbal cold and flu remedies, although research is lacking. Women who are pregnant or breastfeeding should talk to their doctor before taking goldenseal. Goldenseal interacts with a number of medications and should not be taken by people with certain medical conditions. Ask your doctor before taking it.
• Licorice (Glycyrrhiza glabra). Licorice root is a traditional treatment for sore throat, although scientific evidence is lacking. Licorice interacts with a number of medications, including hormonal medications, so ask your doctor before taking it. People with high blood pressure, kidney disease, liver disease, or heart disease, have a history of hormone-related cancer, women who are pregnant or breastfeeding, and those who take blood thinners, such as aspirin, clopidogrel (Plavix), or warfarin (Coumadin) should not take licorice.
• Marshmallow (Althea officinalis). Although there is no scientific evidence that it works, marshmallow has been used traditionally to treat sore throat and cough.
• Peppermint (Mentha x piperita). Like eucalyptus, peppermint is widely used to treat cold and flu symptoms. Its main active agent, menthol, is a good decongestant. Menthol also thins mucus and works as an expectorant, meaning that it helps loosen and break up phlegm. It is soothing and calming for sore throats and dry coughs. DO NOT use peppermint or menthol with infants. DO NOT take peppermint oil by mouth.
• Slippery elm (Ulmus fulva). Slippery elm may help ease a sore throat and has been used traditionally for this purpose, although scientific evidence is lacking. Slippery elm may affect how your body absorbs some medications, so wait at least one hour after taking any other medications before taking slippery elm. Pregnant or breastfeeding women should avoid slippery elm.

Homeopathy

Although very few studies have examined the effectiveness of specific homeopathic therapies, professional homeopaths may consider the following remedies for the treatment of the flu based on their knowledge and experience. Before prescribing a remedy, homeopaths take into account a person's constitutional type. A constitutional type is defined as a person's physical, emotional, and psychological makeup. An experienced homeopath assesses all of these factors when determining the most appropriate treatment for each individual.

• A combination remedy including Aconite, Gelsemium, Eucalyptus, Ipecacuanha, Phosphorus, Bryonia, and Eupatorium perfoliatum.
• A mixture of Anas barbarice hepatis and Cordis extractum.
• Gelsemium. For chills, weakness, lack of energy, fever, and headaches in the back and top of the head; this is one of the most common homeopathic remedies for the flu.
• Eupatorium perfoliatum. For deep aches, sneezing, and coughing.
• Nux vomica. For violent vomiting, irritability, dry cough, chills, and a stuffy nose that develops into a watery, irritating discharge.

Warnings and Precautions

If you are in any of the high risk groups described in the Risk Factors section, be sure to call your doctor at the earliest signs of flu symptoms. The sooner you are treated, the less likely you are to develop complications.

Prognosis and Complications

Most healthy people get over the flu in within 2 weeks. For those at high risk, certain serious, even life threatening complications can occur, including:

• Pneumonia, most common in children
• Encephalitis, an infection of the brain
• Secondary bacterial infection elsewhere in the body

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