Public health officials say the spread of misinformation about the safety and efficacy of vaccines has fueled a series of measles outbreaks across the country. There have been more than 159 cases in 10 states since January.

Testifying before the House Committee on Energy and Commerce, Dr. Anthony Fauci, director of the NIH's National Institute of Allergy and Infectious Diseases, called false information distributed through social media about the use of the Measles, Mumps and Rubella vaccine a 'threat' to public health that could only be counteracted with a sustained public education effort.

"The good news about the internet is that it spreads important information, and that's good, and the bad news about the internet is that when the bad information gets on there, it's tough to get it off," Fauci said.

Dr. Nancy Messonier, director of the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention, said healthcare providers played an important role in helping parents sort through false information about vaccines. He encouraged providers to offer evidence-based information.

"Parents say the person they trust most to help them make healthcare decisions is still their healthcare provider, and that's also true of patients that are hesitant to vaccinate," Messonier said before the committee on Wednesday. "Parents should talk to their healthcare provider who can help sort through the sea of information that's out there."

Fauci said it's hard to refut false information on vaccines because it continues to be disseminated even after it has been discredited or proven false.

"We shouldn't be criticizing people who get this information that's false because they may not know it's false," Fauci said.

Questions whether vaccine misinformation online played a role in this year's measles outbreaks were raised earlier this month after California Rep. Adam Schiff (D) sent a letter to the chief executives of Facebook and Google requesting they address false posts.

"If a concerned parent consistently sees information in their YouTube recommendations that casts doubt on the safety or efficacy of vaccines, it could cause them to disregard the advice of their children's physicians and public health experts and decline to follow the recommended vaccination schedule," Schiff wrote. "Repetition of information, even if false, can often be mistaken for accuracy, and exposure to anti-vaccine content via social media may negatively shape user attitudes towards vaccination."

The measles vaccine is regarded as one of public health's greatest achievements. Before the vaccine was introduced in 1963, as many as 4 million people were infected with measles in the U.S. each year, resulting in about 500 deaths every year.

The disease was declared eliminated by 2000, but cases of measles has been on the rise over the past five years. Health officials claim the spike is caused by clusters of parents who opt to not vaccinate over fears that the vaccine will cause harmful effects, such as autism or encephalitis. Both of those claims have either been discredited or not confirmed.

Nationally measles vaccination coverage remains at 92%, according to the CDC. Rates vary in states like Colorado (88.7%), Washington (90.6%), and Idaho (89.5%), which fall well below the 95% threshold health experts say is needed to maintain what is known as herd immunity. That threshold provides protection to those not able to get immunized.