Flu: Your Top Questions Answered

— -- With the Centers for Disease Control and Prevention reporting a brutal flu season underway, Americans are wondering about the best ways to protect themselves from the illness. Below, your top questions to ABC News about the flu answered by the experts:

Why was the vaccine so ineffective?

This year’s vaccine is only 33 percent effective in preventing the flu because the virus started to “drift” and mutate after the vaccine was already manufactured, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told ABC News.

“The one thing about the flu that you can be sure, it’s really unpredictable,” Fauci said.

Though this year’s vaccine is not a strong match for the most prevalent strain of the flu, experts still recommended people get vaccinated because different strains of the virus will come into play as the season progresses. The shot may be a better match for the subtypes that dominate later in the year.

“There may be some level of cross-protection,” Dr. Richard Besser, ABC News chief health and medical editor, said. ” But we won’t know until March or so until we look back.”

Can I build immunity to a strain of the flu that’s been around before?

When the H1N1 strain hit in 2009, a lot of people got sick, explained Patsy Stinchfield, the director of infection prevention and control with Children's Hospitals and Clinics of Minnesota.

"Since then, everyone got sick or was vaccinated against that particular strain so you hardly see it going around," she said.

Immunity may not be life-long though, Strinchfield added. And even though having any kind of flu can help mitigate future bouts regardless of the subtype, some protection is not total protection.

Other than hand washing, what’s the best way to avoid the flu?

According to the CDC, the single best way to prevent the flu is immunization. Frequent and thorough hand washing can also help prevent the spread. You should also steer clear of sick people when you can, and stay home yourself when you don’t feel well. Because the influenza virus is airborne, take care to cover coughs and sneezes.