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 2 years
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.
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:
When to see a doctor
Most people who get the flu can treat themselves at home and often don't need to see a doctor.
If you have flu symptoms and are at risk of complications, see your doctor right away. Taking antiviral drugs within the first 48 hours after you first notice symptoms may reduce the length of your illness and help prevent more-serious problems.
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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 vaccination, 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 subtypes that can be very different immunologically from what you had before.
Factors that may increase your risk of developing influenza or its complications include:
Age. Seasonal influenza tends to target young children and older adults.
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.
Weakened immune system. Cancer treatments, anti-rejection drugs, corticosteroids and 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, such as asthma, diabetes or heart problems, may increase your risk of influenza complications.
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 BMI of 40 or more have an increased risk of complications from the flu.
If you're young and healthy, seasonal influenza usually isn't serious. Although you may feel miserable while you have it, the flu usually goes away in a week or two with no lasting effects. But high-risk children and adults may develop complications such as:
Pneumonia is the most serious complication. For older adults and people with a chronic illness, pneumonia can be deadly.
The Centers for Disease Control and Prevention (CDC) recommends annual flu vaccination for everyone age 6 months or older.
Each year's seasonal flu vaccine contains protection from the three or four influenza viruses that are expected to be the most common during that year's flu season. This year, the vaccine will be available as an injection and as a nasal spray.
The nasal spray hasn't been available for two years due to questions about its effectiveness. The current version is expected to be effective, according to the CDC. The nasal spray still isn't recommended for some groups, such as pregnant women, children between 2 and 4 years old with asthma or wheezing, and people who have compromised immune systems.
Most types of flu vaccines contain a small amount of egg protein. If you have a mild egg allergy — you get hives only from eating eggs, for example — you can receive the flu shot without any additional precautions. If you have a severe egg allergy, you should be vaccinated in a medical setting and be supervised by a doctor who is able to recognize and manage severe allergic conditions.
Controlling the spread of infection
The influenza vaccine isn't 100 percent effective, so it's also important to take measures such as these to reduce the spread of infection:
Wash your hands. Thorough and frequent hand-washing is an effective way to prevent many common infections. Or use alcohol-based hand sanitizers if soap and water aren't readily available.
Contain your coughs and sneezes. Cover your mouth and nose when you sneeze or cough. To avoid contaminating your hands, cough or sneeze into a tissue or into the inner crook of your elbow.
Avoid crowds. Flu spreads easily wherever people congregate — in child care centers, schools, office buildings, auditoriums and public transportation. By avoiding crowds during peak flu season, you reduce your chances of infection. And if you're sick, stay home for at least 24 hours after your fever subsides so that you lessen your chance of infecting others