HARI SREENIVASAN:

U.S. health officials say the recent measles outbreak that began at Disneyland in Southern California continues to ripple across the nation. Approximately 100 cases have been reported so far.

How great a risk does it pose? And what can be done to stop its spread?

I spoke about this yesterday with Dr. Anthony Fauci. He is the director of the National Institute of Allergy and Infectious Diseases in Washington.

So, Dr. Fauci, we know that there are, what, 20 million measles cases around the world every year. But what does a theme park like a Disneyland do to make sure that the parents and children that are visiting from the United States or from other countries are safe?

DR. ANTHONY FAUCI, Director, National Institute of Allergy and Infectious Diseases: You know, it really relates to the fact that this happens because children are not vaccinated.

The overwhelming number of people who have gotten infected, particularly among the children, are children that have not been vaccinated, because parents, for reasons that are really not based on any scientific data, just don't want their children to be vaccinated.

And it's really unfortunate, because vaccination can prevent all of this.

One of the things we do know about measles is that the vaccine that we have is one of the most effective vaccines we have for any viral disease or for any microbe.

So this all could have been shut down if people had gotten vaccinated. That is the real critical issue.