WASHINGTON (CNN) -- The influenza virus is sweeping across the United States, leaving many people aching, coughing -- and just plain miserable.

Flu cases have been reported in 49 states, with widespread or regional outbreaks in 35, according to the Centers for Disease Control.

Meanwhile, Britain is in the grip of the most serious influenza outbreak in 11 years, health officials said Sunday. An estimated 300 out of every 100,000 Britons have the flu.

In the United States, the flu outbreak has forced hospitals to cancel surgeries to deal with the overflow of flu patients in their emergency rooms.

The Children's National Medical Center in Washington, D.C., "has been mobbed, literally, we are backed up for hours and hours," said Dr. Christina Johns.

The medical facility has "been bombarded 24 hours a day, seven days a week," she said.