More than one-third of United States residents have already been vaccinated against the influenza virus, the Centers for Disease Control and Prevention announced Monday. With National Influenza Vaccination Week, which started last Sunday and ends Saturday, health officials aim to increase that percentage, especially since this year's season may be a bad one.

Influenza—more commonly known as simply "the flu"—is a contagious respiratory illness caused by viruses infecting the nose, throat and lungs. It spreads via infected people coughing, sneezing or talking, though people can also get infected by touching something with the flu virus on it before touching their mouth, eyes or nose.

The 2012-2013 season is shaping up to be one of the worst flu seasons in a while, officials from the CDC said in a teleconference Monday. There have been a larger number of suspected flu cases than usual in five Southern states, and this year's strain may be more virulent. Already, two children have died of the illness.

A similar flu virus struck during the 2003-2004 season, killing more than 48,000 people in one of the most lethal seasons in the past 35 years. Nevertheless, this year's vaccination appears to be better matched to the virus.

"It looks like it's shaping up to be a bad flu season, but only time will tell," CDC Director Dr. Thomas Frieden said.

A jump in the number of influenza cases usually doesn't occur until after Christmas.

“Flu season typically peaks in February and can last as late as May,” Dr. Anne Schuchat, Assistant Surgeon General of the U.S. Public Health Service and Director of CDC’s National Center for Immunization and Respiratory Diseases, said in a news release. “We are encouraging people who have not yet been vaccinated to get vaccinated now.”

More than 200,000 people each year are hospitalized due to complications from the flu, according to the CDC.

Symptoms of the flu include muscle or body aches, headaches, cough, sore throat, fatigue, fever or chills, and vomiting and diarrhea (the latter two are more common in kids). The flu can also worsen chronic medical conditions or cause death.

People are contagious a day before symptoms appear and up to a week after getting sick.

The CDC recommends getting annual vaccines as early as possible, as it takes a few weeks to reach full immunity.

In St. Louis, the number of cases of flu has been on the rise—more so than usual.

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—Melanie Rosen contributed to this report.

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