Influenza isn't usually tested for or reported so the CDC has to look indirectly for evidence of flu. Data for last week show states in every U.S. region have extra cases of flu-like illness, which can include colds, bacterial infections and a range of respiratory viruses. But anywhere from 11 to 36 percent of people who are tested turn out to have influenza.
It's worst in the Southeast, New Jersey and parts of the Midwest.
CDC's been warning of a bad flu season. Most infections are caused by a strain called H3N2 this year, and two-thirds of them are a strain that is not included in this year's flu vaccine.
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There are also a few cases of H1N1 and two types of influenza B. So at least four strains of flu are circulating now. Some are included in the current flu vaccines and some are not. CDC says it is still not too late to get vaccinated and advises that it will protect people from some of the flu that's out there.
Last year, flu started hitting epidemic levels in the second week of January, and the year before the epidemic started right around New Year.
And like every year, flu is killing children.
"A total of 15 influenza-associated deaths have been reported during the 2014-2015 season from nine states (Arizona, Colorado, Florida, Minnesota, North Carolina, Nevada, Ohio, Texas, and Virginia," CDC said in a statement. About half the children sick enough to be hospitalized had been perfectly healthy. Many of those with so-called underlying conditions were obese or had asthma, both of which can raise the risk that someone will get seriously ill from flu.
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Flu usually hits the very young and the very old the hardest. Depending on the season, it kills anywhere between 4,000 and 50,000 people a year in the United States.
Last year, the main circulating strain was H1N1 flu, which first showed up in 2009 when it caused a new pandemic. And like in 2009, it hit people under 65 the hardest. Flu killed at least 100 children last year.
The best way to protect yourself from any flu virus is to wash your hands frequently. Flu is transmitted by close contact with someone sneezing or coughing or by touching those little droplets and then putting your hands into your eyes, nose or mouth.