A measles outbreak in Orange County in California, affecting 21 people is the most recent example of the return of the highly contagious viral illness.

“There have been 49 cases confirmed in the state so far this year,” Dr. Nancy Snyderman, NBC News’ chief medical editor told TODAY’s Natalie Morales. “Eight big pockets are accounting for 77 percent of cases in this country and there are two reasons: under-vaccinated children or unvaccinated children.”

Measles is a respiratory infection that causes fever, runny noses, and a rash. In some cases, it can lead to death. Reports of measles, once thought to be eradicated in the United States, have been increasing across the country and experts expect that more outbreaks will be seen. Officials in New York City and San Francisco have also seen cases.

Trending stories,celebrity news and all the best of TODAY. This site is protected by recaptcha

Vaccinated people can carry the disease and when they come into contact with unvaccinated people these people contract the illness. Ninety percent of the time unvaccinated people get the illness, said Dr. Greg Wallace, Head of the Centers for Disease Control Measles, Mumps, Rubella, and Polio team.

“It is the most contagious of the vaccinated diseases,” Wallace said.

Because there are few effective treatments for measles, vaccines are the best way to prevent it. Children should receive the measles, mumps, and rubella (MMR) vaccine between 12 to 15 months of age and a booster before they enter school between the ages of 2 to 4 years old, Wallace and Snyderman recommend. If adults are working with children or in health care or traveling — especially if they were born before 1989 when the second shot was required — they might want to receive a booster for additional protection.

“It’s certainly effective and it is certainly safe, but there can be side effects. Some small percentage, less than 10 percent, may get a mild fever and even a rash,” Wallace said, adding that the illness is far worse than the side-effects of the vaccine.

“The vaccine does work and these outbreaks don’t occur unless you have enough people who have not been vaccinated.”

If people suspect they have measles they should call the doctor’s office or emergency room ahead of time so the staff can prepare to isolate them to prevent further spread of the illness, Wallace said.

While there have not been any deaths in the United States due to measles in several years, Wallace said people died during an outbreak in France in 2011. Both Wallace and Snyderman stress the importance of vaccination.

“I want parents to be a little scared because 15 years ago, we thought we eradicated measles in this country. And we've seen an uptick. We shouldn’t fool ourselves,” Snyderman said.