Influenza, a viral infection, causes an illness that can range from mild to life-threatening.

Influenza, more commonly known as the flu, is a viral infection of the respiratory tract that affects the nose, throat, and sometimes lungs.

Outbreaks of flu tend to happen annually, at about the same time every year. However, each outbreak may be caused by a different subtype or strain of the virus, so a different vaccine is needed to prevent the flu each year.

For most people, a bout of flu is an unpleasant but short-lived illness. For others, however, flu can pose serious health risks, particularly if complications such as pneumonia develop.

Every year, thousands of Americans die from the flu. According to the Centers for Disease Control and Prevention (CDC), the number of deaths caused annually by flu in the United States ranged from 3,000 to 49,000 between 1976 and 2006, with an annual average of 23,607 flu-related deaths.

The best way to avoid getting the flu is to get an annual vaccination, encourage the people you live and work with to do likewise, stay away from people who are sick, and wash your hands frequently.

Types of Flu

There are three types of human influenza virus, type A, type B, type C, and many different variants within those types.

Because several types or strains of flu can circulate simultaneously, each year's flu vaccine protects against the three or four viruses predicted to be most common in the coming season.

Influenza A and B cause the seasonal epidemics of flu, with symptoms that can range from mild to severe. Two types of influenza A and one or two influenza B viruses are included in the seasonal vaccine.

Common signs and symptoms of the flu include:

• Fever
• Aching muscles
• Chills and sweats
• Headache
• Dry, persistent cough
• Shortness of breath
• TiRedness and weakness
• Runny or stuffy nose
• Sore throat
• Eye pain
• Vomiting and diarrhea, but this is more common in children than adults

Influenza viruses travel through the air in droplets when someone with the infection coughs, sneezes or talks. You can inhale the droplets directly, or you can pick up the germs from an object — such as a telephone or computer keyboard — and then transfer them to your eyes, nose or mouth.

People with the virus are likely contagious from about a day before symptoms appear until about five days after they start. 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've had influenza in the past, your body has already made antibodies to fight that specific strain of the virus. If future influenza viruses are similar to those you've encountered before, either by having the disease or by getting vaccinated, those antibodies may prevent infection or lessen its severity. But antibody levels may decline over time.

Also, antibodies against influenza viruses you've encountered in the past may not protect you from new influenza strains that can be very different viruses from what you had before.

Usually, you'll need nothing more than rest and plenty of fluids to treat the flu. But if you have a severe infection or are at higher risk of complications, your doctor may prescribe an antiviral drug to treat the flu. These drugs can include oseltamivir (Tamiflu), zanamivir (Relenza), peramivir (Rapivab) or baloxavir (Xofluza). These drugs may shorten your illness by a day or so and help prevent serious complications.

Oseltamivir is an oral medication. Zanamivir is inhaled through a device similar to an asthma inhaler and shouldn't be used by anyone with certain chronic respiratory problems, such as asthma and lung disease.

Antiviral medication side effects may include nausea and vomiting. These side effects may be lessened if the drug is taken with food.

Most circulating strains of influenza have become resistant to amantadine and rimantadine (Flumadine), which are older antiviral drugs that are no longer recommended.

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

• Age. Seasonal influenza tends to target children 6 months to 5 years old, and adults 65 years old or older.
• Living or working conditions. People who live or work in facilities with many other residents, such as nursing homes or military barracks, are more likely to develop the flu. People who are staying in the hospital also are at higher risk.
• Weakened immune system. Cancer treatments, anti-rejection drugs, long-term use of steroids, organ transplant, blood cancer or HIV/AIDS can weaken the immune system. This can make it easier to catch the flu and may also increase the risk of developing complications.
• Chronic illnesses. Chronic conditions, including lung diseases such as asthma, diabetes, heart disease, nervous system diseases, metabolic disorders, an airway abnormality, and kidney, liver or blood disease, may increase the risk of influenza complications.
• Race. Native American people may have an increased risk of influenza complications.
• Aspirin use under age 19. People who are younger than 19 years of age and receiving long-term aspirin therapy are at risk of developing Reye's syndrome if infected with influenza.
• Pregnancy. Pregnant women are more likely to develop influenza complications, particularly in the second and third trimesters. Women are more likely to develop influenza-related complications up to two weeks after delivering their babies.
• Obesity. People with a body mass index (BMI) of 40 or more have an increased risk of flu complications.

You'll need nothing more than rest and plenty of fluids to treat flu. But if you have a severe infection or are at higher risk of complications, your doctor may prescribe an antiviral drug to treat the flu. The flu treatment you should take depends on your symptoms.

• Nasal or sinus congestion: These are common symptoms of flu, and to treat this condition, a decongestant can be helpful. Decongestants come in oral or nasal spray forms and are used to reduce swelling in the nasal passageways. Pseudoephedrine and phenylephrine are commonly used oral decongestions.
• Runny nose, postnasal drip, or itchy, watery eyes: If you have a runny nose, postnasal drip, or itchy, watery eyes, then antihistamines may be helpful. Antihistamines block the effect of histamines and help relieve annoying symptoms such as sneezing, itching, and nasal discharge.
• Fever and body ache: Acetaminophen (Tylenol) or medicines like ibuprofen (Advil, Motrin) and naproxen (Aleve) are over-the-counter options for fever and pain relief. Children under 19 should avoid aspirin.
• Sore throat: Over-the-counter pain relievers and medicated lozenges and gargles can temporarily soothe a sore throat.
• Antiviral flu drugs like baloxavir marboxil (Xofluza), oseltamivir (Tamiflu), peramivir (Rapivab), or zanamivir (Relenza) are taken to decrease the severity and duration of flu symptoms. They may also be used to prevent flu.

Symptoms
Fever,Cough,Sore throat,Runny or stuffy nose,Muscle or body aches
Conditions
Fever,Weakness,Severe fatigue,Warm, flushed skin,Chills,Headache
Drugs
Oseltamivir (Tamiflu),Zanamivir (Relenza),Peramivir (Rapivab) or baloxavir (Xofluza),Pseudoephedrine and phenylephrine are commonly used oral decongestions