Measles was declared “eliminated” from the U.S. 15 years ago by the U.S. Centers for Disease Control and Prevention but recent outbreaks have health experts concerned that the disease could make a more permanent return to the U.S. if vaccination rates fall.

Dr. Stephen Morse, an infectious disease expert at Columbia University Mailman School of Public Health, said for measles to become permanent -- that is, become "endemic" -- again to the U.S., measles immunizations would have to drop below 90 percent.

“It is highly contagious,” Morse said of measles, noting that every infected person could infect another 10 to 20 non-immune people. “You could have sporadic cases anytime [immunization levels] fall below something that approaches 90 to 95 percent.”

Currently the national immunization rate for measles is 91.9 percent as of 2013, according to the CDC. However during this time pockets of unimmunized people have helped fan recent outbreaks, the agency noted.

“The concern is that some groups are opting out of vaccination,” the CDC said in a statement to ABC News. Over time they become “susceptible to outbreaks despite high national vaccine coverage levels.”

In the last month, a measles outbreak has infected at least 102 people in 14 states, according to U.S. health officials. Last year, 644 people were infected with the virus in various outbreaks.

Before the measles vaccine was introduced in the U.S., approximately 3 to 4 million people were infected with the virus every year, nearly 50,000 were hospitalized and 400 to 500 people died of the virus, according to the CDC.

The disease was considered eliminated by the CDC in 2000 due to an absence of continuous transmission of the disease over 12 months.

While the recent outbreaks are a tiny fraction of the total annual cases in the U.S. before the vaccine was introduced, Morse said they could signal a worrisome trend if there is an increase in pockets of people who have not been immunized.

If enough people are not protected, the virus could be re-introduced from abroad and then continuously circulate throughout the country, meaning it has become endemic again in the U.S.

Morse, who had measles as a child, said he hoped parents on the fence about vaccines would reconsider in the wake of the latest outbreak.

"I would say there’s no comparison between having the disease and having the vaccine," said Morse. "Even in the best case, [measles] really knocks you out."

The CDC currently recommends two shots to protect against measles -- one at 12 to 15 months and another between the ages of 4 to 6 years. One shot will provide 95 percent protection and two will provide 99 percent protection, according to the CDC.

The virus remains endemic to many other parts of the world, including Africa, Europe and parts of Asia. In these areas, measles remains a deadly problem, killing on average 400 people every day across the globe, according to the World Health Organization.