First, a flu shot is not a guarantee that you won't get the flu, though it's a good step to take, especially in years when vaccine is plentiful.

Second, influenza is only one of the viruses lurking around. And there is no vaccine to protect us against all the others.

What is influenza?

Influenza is caused by the influenza virus, which infects the respiratory tract. Unlike many other viral respiratory infections, such as the common cold, the flu can cause severe illness and life-threatening complications in many people.

What are the symptoms?

Influenza is a respiratory illness. Symptoms of flu include fever, headache, extreme tiredness, dry cough, sore throat, runny or stuffy nose, and muscle aches. Children can have additional gastrointestinal symptoms, such as nausea, vomiting and diarrhea, but those are uncommon in adults. Although the term "stomach flu" is sometimes used to describe vomiting, nausea or diarrhea, these illnesses are caused by other viruses, bacteria or parasites, and are rarely related to the flu.

When is the flu season?

The peak of the U.S. flu season can occur anywhere from late December to March.

How does the flu spread?

The main way that influenza viruses are spread is from person to person in droplets of coughs and sneezes. This can happen when droplets from the cough or sneeze of an infected person are propelled (generally up to 3 feet) through the air and deposited on the mouth or nose of people nearby. Though much less frequent, the viruses also can be spread when a person touches respiratory droplets on another person or an object and then touches their own mouth or nose before washing their hands.

If I got the flu last year, will I have immunity this year?

In general, a person who is infected with an influenza virus one year will have some immunity to closely related viruses that may persist for one or more years. The degree of protection depends on the health of the person involved. Young and healthy people with normal immune systems will likely have good immunity against the same or related strains of virus from one year to the next.

It's important to remember that there are different types of influenza viruses circulating and different variants within virus types, and the same type of flu virus does not necessarily circulate each year.

Does the flu have complications?

Yes. Some of the complications caused by flu include bacterial pneumonia, dehydration and worsening of chronic medical conditions, such as congestive heart failure, asthma or diabetes. Children may get sinus problems and ear infections. People over 65 and people of any age with chronic medical conditions are at highest risk for serious complications related

to the flu.

How do I find out if I have the flu?

A test can confirm that an illness is influenza if the patient is tested within the first two to three days after symptoms begin. In addition, a doctor's examination may be needed to determine whether a person has another infection that is a complication of influenza.

How soon will I get sick if I am exposed to the flu?

The time from when a person is exposed to flu virus to when symptoms begin is about one to four days.

How long is a person with flu virus contagious?

The period when an infected person is contagious depends on the age of the person. Adults may be contagious from one day prior to becoming sick and for three to seven days after they first develop symptoms. Some children may be contagious for longer than a week.