"It is troubling, because this was completely preventable," Dr. Amesh Adalja, an infectious disease physician and senior scholar at Johns Hopkins, told CBS News on Wednesday. "What we're seeing is, one of the places in Texas -- it has the lowest vaccination rates, the highest school exemption rates from measles vaccination -- having a measles outbreak, including hospitalizations of individuals who've been infected with measles."

Adalja said this should act as a reminder that measles is still a risk when proper precautions aren't taken.

"It's the most contagious infectious disease known to humans. And when we start to see measles outbreak, that's a sign that there is a chink in the armor of vaccination and the fact that it's preventable, I think, is what people should understand. This doesn't have to happen if we can get vaccination rates back to high levels again."

"If somebody is fully vaccinated against measles and has a normal immune system, they don't need to worry about measles," said Adalja. "You're not going to be at risk for contracting measles because the vaccine the vaccine is very, very effective."

One dose of the MMR vaccine, which protects against measles, mumps and rubella, is 93% effective against measles and the recommended two doses is 97% effective, research shows.

Studies have shown the MMR vaccine, which was first introduced in 1971, may be given safely and effectively in combination with other vaccines.