Influenza

Influenza (flu) is a contagious respiratory illness caused by influenza viruses. The viruses pass through the air and enter your body through your nose or mouth. Flu can cause mild to severe illness. Serious outcomes of flu infection can result in hospitalization or death. Some people, such as elderly people, newborns and young children, and people with certain health conditions, are at high risk for serious flu complications. Between 5% and 20% of people in the U.S. get the flu each year. The best way to prevent the flu is by getting vaccinated each year.

Symptoms of the flu come on suddenly and are worse than those of the common cold. Colds rarely cause a fever or headaches. Flu almost never causes an upset stomach. And "stomach flu" is not really flu at all, but gastroenteritis.

Most people with the flu recover on their own without medical care. People with mild cases of the flu should stay home and avoid contact with others, except to get medical care. If you get the flu, your health care provider may prescribe medicine to help your body fight the infection and lessen symptoms.

The main way to keep from getting the flu is to get a yearly flu vaccine. Good hygiene, including hand washing, can also help.

Flu viruses travel through the air in droplets when someone with the infection coughs, sneezes or talks. People with flu can spread it to others up to about 6 feet away. You can inhale the droplets directly, or you can pick up the germs from an object 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 5 days after symptoms begin, though sometimes people are contagious for as long as 10 days after symptoms appear. 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 have 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 have encountered before, either by having the disease or by vaccination, those antibodies may prevent infection or lessen its severity. Antibodies against flu viruses you have encountered in the past cannot 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. Flu tends to occur more frequently in young children and older adults.
• Living conditions. People who live in facilities along with many other residents, such as nursing homes or military barracks, are more likely to develop the flu.
• Weakened immune system. Cancer treatments, anti-rejection drugs, corticosteroids and HIV/AIDS can weaken your immune system, which makes 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.
• Obesity. People who are extremely overweight or obese have an increased risk of complications from flu.

Most people recover with rest and fluids. See your doctor early if you are high-risk (older adults, young children, pregnant, or with chronic illness), and seek emergency care for trouble breathing, chest pain, confusion, or symptoms that improve then return with fever.