Influenza is a viral infection that attacks your respiratory system — your nose, throat and lungs. Influenza, commonly called the flu, is not the same as the stomach “flu” viruses that cause diarrhea and vomiting.
Influenza and its complications can be deadly. People at higher risk of developing flu complications include:
- Young children
- Adults older than 65
- Pregnant women
- People with weakened immune systems
- People who have chronic illnesses
Your best defense against influenza is to receive an annual vaccination.
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 before symptoms first appear until five to 10 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 are very different immunologically from what you had before. A number of virus subtypes have appeared in humans since the global epidemic (pandemic) of 1918, which killed tens of millions of people.
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 F (38 C)
- Aching muscles, especially in your back, arms and legs
- Chills and sweats
- Dry cough
- Fatigue and weakness
- Nasal congestion
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.