KNOXVILLE, Tenn. (WVLT) -- An outbreak of more than 120 cases of measles in the United States is drawing concern as the largest outbreak in the last decade.

Officials believe it is related to travelers coming back to the United States from other countries.

Many people in East Tennessee grew up getting the measles vaccine and that was that.

But health officials are now getting adolescents to get a second dose, and if you're in a high risk category, including international travelers, then you should get a second dose, too.

Most Americans have been vaccinated and are not familiar with the once common disease.

University of Tennessee infectious disease consultant Dr. Mark Rasnake says, “Measles is one of the classic viral diseases of childhood. It typically causes a rash and high fever in children."

Measles cases have been very low since vaccinations came out three to four decades ago.

Dr. Rasnake says measles in adults can be much more serious, even deadly.

"The manifestations of measles can sometimes be more serious in an adult. They can develop terrible pneumonia and other complications."

When the vaccine came out, a single dose was administered to very young children for what they thought would last a lifetime.

But in the mid 1990's, that theory changed when there was a large outbreak among college age students.

Dr. Rasnake says, "At that point, they began re-vaccinating during adolescence to give a second booster dose and now that's the standard of care."

But that leaves many middle age adults with only a single dose. With this latest outbreak, should people begin running for a second dose? Dr. Rasnake says not so fast.

"Only those that would have a particular risk of contacting measles go back and get that second dose of vaccine and that's if you're a health care worker or if you're traveling abroad to places with measles activity."

Over the years, some parents have had some concerns about links between the vaccine and autism. Dr. Rasnake says you really shouldn't.

"At this pint the protection offered by the measles vaccine exceeds any even theoretical small increase in autism that's out there."

Dr. Rasnake says adults born before 1957 probably do not need to get the vaccine.

He says most of that age group had measles and are now immune from getting it again for life.

Dr. Rasnake says measles are simply treated with prescribed doses of Vitamin A.