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Influenza, or flu, is a virus that targets the respiratory system. Here are answers to the 10 most common questions about the flu.

1. What is the difference between a cold and the flu?

The flu and the common cold are both respiratory illnesses, but they are caused by different viruses. Influenza or "the flu" develops when a flu virus infects the respiratory system, including your nose, throat, bronchial tubes, and possibly the lungs. A cold virus usually infects only the upper respiratory tract: your nose and throat.

The flu usually causes more severe illness than the common cold. Flu can bring on fever, body aches, and exhaustion, symptoms that are rarely caused by simple colds.

2. What are flu symptoms and when is a person contagious?

Primary symptoms of flu are fever, fatigue, body aches, chills, headache, sore throat, and cough. The cough is a bronchial tube irritation and is usually not productive -- you're not coughing up gunk. The flu is usually at its worst for three to four days. The cough may linger longer. Recovery may take seven to 10 days. You may have lingering fatigue for several weeks.

There's one catch with flu viruses. About 24 to 72 hours after you're infected, you become contagious. Yet you may not have symptoms, so you don't know you're sick. You feel completely healthy and go about your daily affairs -- spreading the virus wherever you go.

Stay at home while you have the flu, and for at least 24 hours after your fever is gone without the use of fever-reducing medicine. Once your fever is gone for a day, you're no longer contagious and can return to work or school. Also, you will recover more quickly if you get some rest.

3. What's the best treatment for flu?

There's no single "best" treatment for flu, but there are many ways you can ease symptoms.

Prescription flu drugs can shorten the time you feel sick if taken when symptoms first appear. They work best when taken within 48 hours of symptoms, but they may also prevent severe disease if taken more than 48 hours after the first symptoms. Over-the-counter cold and flu drugs can offer some relief from fever, aches, stuffy nose, and cough. They don't "cure" the flu, but may help keep you more comfortable.