Ten states reported widespread flu activity this week, mostly in the Southeast, but flu is now fanning across the rest of the nation, according to the weekly report issued by the Centers for Disease Control and Prevention.

And it's a once-familiar strain - the H1N1 virus that plagued the country in 2008-09. "It's our old swine flu friend and it's causing over 95 percent of the cases," said Dr. William Schaffner, infectious disease specialist at Vanderbilt University.

"There is a lot of flu going on right now," he said. "Philadelphia has been reporting a substantial amount and it's gaining steam and moving across the country."

During week 51, the CDC reports widespread flu in Alabama, Alaska, Kansas, Louisiana, Massachusetts, New York, Pennsylvania, Texas, Virginia and Wyoming. High levels like those in the Southeastern states have not been seen at this point in the flu season for about four years.

The CDC is reporting four baby deaths, compared with more than 100 last year.

"So far we are doing very well with them," said Schaffner. "The concern has come through email exchanges among infectious disease doctors. This strain has a predilection for young adults. Any number have been admitted with very serious illness to intensive care units."

"Some have underlying conditions," he said. "But others are perfectly healthy young adults."

The dominant H1N1 strain is a "tight match" with the vaccine, according to Schaffner.

Flu shots are recommended for anyone over the age of 6 months. And it's not too late to get a flu shot, say experts.

"But don't linger," said Schaffner. "Run. Do not walk to your pharmacy, clinic, doctor, hospital or public health department and get this now - as a New Year's present."