The recent debate surrounding vaccinations in the United States has drawn a lot of attention to the people who are against vaccinations. Who are these people, exactly?

Well, it turns out they are young Americans who only really know a country where measles almost never occurred (until last year).

Americans are largely in agreement that the measles vaccination is safe. The Pew Research Center released numbers  Monday showing that 83 percent of Americans feel this way, with just 9 percent of people saying they felt the vaccination is unsafe. (Another 7 percent said they just don’t know.)

When you break it down by age group, though, you find out a little more about who, exactly, makes up the 9 percent category. Pew reported that Americans age 18 to 29 are significantly more likely than their parents and grandparents to say that vaccinations are not safe. Fifteen percent of younger Americans in this age bracket say they think the vaccinations are unsafe, more than double the number of Americans ages 50 or older.

This isn’t a galloping shock. As we noted last week (when Pew released different numbers), millennials are also much more likely than older generations to think vaccination should be a choice. This comes from experience, or lack thereof; measles used to be a widespread problem in this country, infecting millions of Americans (between 400 and 500 of whom died) each year until the early 1960s. That is when the vaccination became widespread.

This vaccine was so successful that by 2000, measles was deemed eliminated in the United States. In the years that followed, measles was barely a blip on the public-health radar; in any given year, there were between 37 and 220 cases, though the number mostly stayed in the double-digits. The situation changed last year, when there were 644 cases.

In other words, if you just turned 65, you reached your teenage years knowing how severe measles could be. If you just turned 29, you became a teenager when the disease was so rare it was declared eliminated. And over the last half-century, older Americans have seen the dramatic shift in measles contraction in this country. They have seen their children and grandchildren get vaccinated, and they have seen the disease recede. Meanwhile, over the last 15 years, younger Americans have heard from celebrities and other voices questioning a vaccination that, to them, attacks a problem they never really had reason to understand.

Meanwhile, the Centers for Disease Control and Prevention reported Monday that the outbreak has now spread to three additional states. There are currently 121 known cases in 17 states and the District of Columbia.