Influenza is a viral infection that attacks your respiratory system — your nose, throat and lungs. Influenza is commonly called the flu, but it's not the same as stomach "flu" viruses that cause diarrhea and vomiting.
For most people, influenza resolves on its own. But sometimes, influenza and its complications can be deadly. People at higher risk of developing flu complications include:
- Young children under age 5, and especially those under 12 months
- Adults older than age 65
- Residents of nursing homes and other long-term care facilities
- Pregnant women and women up to two weeks postpartum
- People with weakened immune systems
- People who have chronic illnesses, such as asthma, heart disease, kidney disease, liver disease and diabetes
- People who are very obese, with a body mass index (BMI) of 40 or higher
Though the annual influenza vaccine isn't 100 percent effective, it's still your best defense against the flu.
Symptoms:
Initially, the flu may seem like a common cold with a runny nose, sneezing and sore throat. But colds usually develop slowly, whereas the flu tends to come on suddenly. And although a cold can be a nuisance, you usually feel much worse with the flu.
Common signs and symptoms of the flu include:
- Fever over 100.4 F (38 C)
- Aching muscles
- Chills and sweats
- Headache
- Dry, persistent cough
- Fatigue and weakness
- Nasal congestion
- Sore throat
Causes
Flu 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 the day or so before symptoms first appear until about five days after symptoms begin. 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 particular 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 antibodies against flu viruses you've encountered in the past can't protect you from new influenza strains that can be very different immunologically from what you had before.
Risk factors
Factors that may increase your risk of developing influenza or its complications include:
- Age. Seasonal influenza tends to target children younger than 12 months of age 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 influenza. People who are hospitalized are also 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 your immune system. This can make it easier for you to catch influenza and may also increase your risk of developing complications.
- Chronic illnesses. Chronic conditions, including lung diseases such as asthma, diabetes, heart disease, neurological or neurodevelopmental disease, an airway abnormality, and kidney, liver or blood disease, may increase your 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 who are up to two weeks postpartum also are more likely to develop influenza-related complications.
- Obesity. People with a body mass index (BMI) of 40 or more have an increased risk of complications from the flu.
Diagnosis
Your doctor will conduct a physical exam, look for signs and symptoms of influenza, and possibly order a test that detects influenza viruses.
During times when influenza is widespread, you may not need to be tested for influenza. Your doctor may diagnose you based on your signs and symptoms.
In some cases, your doctor may suggest that you be tested for influenza. He or she may use various tests to diagnose influenza. Polymerase chain reaction (PCR) testing is becoming more common in many hospitals and labs. This test may be done while you're in your doctor's office or in the hospital. PCR testing is more sensitive than other tests and may be able to identify the influenza strain.
Treatment
Usually, you'll need nothing more than bed rest and plenty of fluids to treat the flu. But if you have severe infection or are at higher risk for complications, your doctor may prescribe an antiviral medication, such as 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.
