About a decade ago, health officials declared an "end" to measles in the country. Now, that has changed: 131 cases of measles have been reported so far this year, more than three times the number in 2007.

In all 15 patients have been hospitalized as a result of measles since January last. They suffered symptoms such as high fever, dehydration and pneumonia. Four of those who were hospitalized for measles were infants.

Viral disease expert Dr. Jane Seward says the developments are indeed a reminder that Americans are still at risk for measles, an easily transmissible airborne virus.

"Measles, although it's not transmitting in this country, it's just a plane ride away," says Seward, deputy director for the division of viral diseases at the National Center for Immunization and Respiratory Diseases with the Centers for Disease Control and Prevention.

"There are 20 million cases in the world, and any time people travel who are not protected through vaccination, they can bring it back if they go to a country where measles is occurring," Seward said in an interview to NPR, a prominent radio station.

Or, someone who's already infected can come to this country and spread the disease.

Either way, Seward says, the virus is increasingly finding its way to vulnerable unvaccinated populations — "mainly children whose parents have chosen not to vaccinate."