Why did the flu hit with such a vengeance this winter? Experts at the Centers for Disease Control point to a phenomenon calld antigenic drift.

Antigenic drift is the ability of viruses to continually change their genetic material. As a result, the proteins that coat the virus also are altered The human immune system doesn't immediately recognize these altered viruses, and the body's attack on the viral invaders is delayed. The greater the change, the longer it takes for the immune system to kick into action.

This year, about three quarters of the flu cases typed have been of the B variety, the CDC reports. This is a change from the previous year, when type A was the most prevalent. In addition, this year's type B variety was dramatically altered from past type B viruses.

"The type B viruses have drifted," says Karl Kappus, an epidemiologist with the CDC's influenza branch. "This year, type B has moved along fairly quickly compared with what it does sometimes. And we think that is why there has been so much type B activity."

Type B infections "have also been relatively more prevalent in younger people," Kappus says. "It was surprising to see healthy people in their twenties, thirties and forties really laid out this year, not just for a day or so but for a longer period."

When the virus changes, he says, "even though almost all adults have been previously infected with type B influenza, we get real illness."