(CNN) -- The number of states reporting widespread seasonal flu activity jumped from four to 10 last week, the Centers for Disease Control and Prevention said Friday.
Widespread activity was reported in Alabama, Alaska, Kansas, Louisiana, Massachusetts, New York, Pennsylvania, Texas, Virginia and Wyoming, according to the CDC's weekly flu advisory report, covering the week ending December 21.
The previous week, only Alabama, Louisiana, New York and Texas reported widespread flu.
"Widespread" means that more than 50% of geographic regions in a state -- counties, for example -- are reporting flu activity. It addresses the spread of the flu, not its severity.
However, six states -- Alabama, Louisiana, Mississippi, Missouri, Oklahoma and Texas -- reported a high proportion of outpatient visits to health care providers for flu-like illnesses.
So far, "it's a typical influenza season, if I can use that word," said Dr. Michael Jhung, a medical officer in the CDC's flu division.
The season usually begins in the winter months and peaks in January or February.
The only atypical thing seen this year is that the most common strain has been H1N1, which became known as swine flu during a 2009 outbreak.
"It's the same virus that we saw in 2009 that caused the pandemic," Jhung said. At the time, it was called swine flu since it was seen for the first time in humans.
But since then, "it's established itself very nicely in the human population," Jhung said. "We've seen it every season since 2009 in people." The virus is no longer referred to as swine flu but instead as a human seasonal virus.
The strain is so common that it's included in the flu vaccine, including this year's vaccination, he said.
Four pediatric flu deaths have been reported since September 29, according to the CDC. The agency does not track adult deaths related to the flu, although some states do, and deaths have been reported.
The exact number of flu-related adult deaths is hard to track and varies from year to year. The CDC has estimated that from 1976 through 2007, between 3,000 and 49,000 people died of flu-related causes.
"It depends on the season; it depends on the virus," Jhung said.
Last year, 381,000 people were hospitalized and 169 children died in what's being called a relatively severe season.
However, the CDC estimates that flu vaccination prevented 6.6 million illnesses last year, 3.2 million doctor visits and at least 79,000 hospitalizations.
Flu vaccines are recommended for everyone 6 months and older, especially pregnant women and those at high risk of complications, including the elderly, children younger than 5 years and those with underlying medical conditions such as asthma or diabetes.
Antiviral medications are a good treatment if you do get sick, Jhung said, particularly those at high risk for complications. Ideally, antivirals should be started within two days of when symptoms appear.
CNN's Caleb Hellerman contributed to this report.