Influenza is a contagious respiratory illness caused by viruses of the same name. Flu season generally begins in November, and an estimated 10 percent to 20 percent of Americans come down with it each year.

The flu can overtake an area quickly. It spreads somewhat easily from person to person. The disease moves primarily through respiratory droplets of coughs and sneezes. Those droplets can travel up to 3 feet and still infect someone.

It takes one to four days - on average two days - for a person exposed to the flu virus to develop symptoms.

The best prevention for individuals, particularly those at high risk for its serious complications, is to get a flu shot. It is particularly recommended for the elderly and children 6 months to 23 months.

You can still get the flu even if you got a flu shot this year, although the shot often lessens the severity of the virus and can prevent deadly complications. Typically, the flu shot protects between 70 percent and 90 percent of healthy people under age 65. The elderly are more susceptible.

The power of the flu shot also depends on how well it matches the flu virus in circulation.

Colds usually begin slowly and last only two to seven days, although it can be as long as two weeks. They start with a scratchy, sore throat, followed by sneezing and a runny nose. You may get a mild cough later. Infants and young children can sometimes run temperatures up to 102 F. with a cold.

Flu often begins with a sudden headache and dry cough, possibly a runny nose and sore throat; also achy muscles and extreme fatigue. You may run a fever up to 104. Most people feel better in a couple of days, but the tiredness and cough can last for two weeks or longer. Flu can cause severe illness and life-threatening complications in some people.

Children may have symptoms - nausea, vomiting or diarrhea - that are not common for adults.

Flu can be confirmed with a test if given within two to three days after symptoms begin, but getting it isn't always practical.

A combination of symptoms - sustained fever and chills, chest pain that gets worse when taking a deep breath and sputum that's a yellow color - can indicate pneumonia and a doctor should be consulted.

They include bacterial pneumonia, dehydration and worsening of chronic medical conditions such as congestive heart failure, asthma or diabetes. Seniors and those with chronic medical conditions are at highest risk.

It's estimated 10 percent to 20 percent of U.S. residents get the flu and 114,000 are hospitalized each season for flu-related complications. Typically, it kills about 36,000 people in the United States each year.

A major mutation of a flu strain usually occurs every 10 years and can cause a flu pandemic - a worldwide outbreak. The last time there was a mutation similar to this year's Fujian strain, nearly 65,000 people died - and that was only five years ago. Still, while health officials say the world is overdue for another flu pandemic, there is no sign of that happening this season.

Sources: Associated Press, Centers For Disease Control and Prevention, World Health Organization, Food and Drug Administration