Dismissing influenza as an inevitable risk of winter could be a fatal mistake, say doctors who warn that flu kills an average of 20,000 Americans a year.

"The reason people die from the flu is it hits the body so hard," said Dr. John D. Nicolas, a clinical medicine instructor at Northwestern University and head of a seminar Thursday on prevention of colds and flu."The most common misconception is, it's not serious," said Dr. Walter Gunn, an epidemiologist at the Centers for Disease Control in Atlanta.

Most who succumb are elderly or people with chronic health problems. They should be vaccinated now to be ready for the flu season, usually beginning in December. In some years flu claims twice or more the annual rate of 20,000.

"Because of people getting together for the Thanksgiving holiday, traveling from other places, spending a few days together inside the house, that's a perfect environment for spreading the virus around," said Mary Huck, spokeswoman for the Illinois Department of Public Health.

Flu, or influenza, is spread by virus-infected droplets coughed or sneezed into the air. Victims develop fever, headache, muscle ache and fatigue.

The vaccine is effective in 70 percent to 90 percent of people under 65 and about half the people over 65, Gunn said. It can protect 85 percent of the elderly from dying of flu or its complications because its symptoms will be milder even if they catch it after getting the vaccine, he said.

Yet only 30 percent to 40 percent of high-risk people get immunized, he said, possibly because they fear becoming infected from the vaccine.

"You can't get flu from the shot. The virus (in it) has been killed," Gunn said.

The vaccine protects against the three strains of flu that health authorities have guessed will be the most prevalent. It changes every year and is good for one year.