PORTLAND, Ore. — Herd immunity is what happens when a community has an immunization rate of 90 percent or more: both the individual and community are safer from disease.

Because so much of the population is immunized the people who cannot be immunized, like children who are too young and people with compromised immune systems, will be protected.

As of Saturday, there are 31 confirmed cases of measles in Clark County and an additional nine suspected cases.

Latest on Clark County measles outbreak

Clark County has not had herd immunity since 2004-2005.

According to the Washington State Department of Health, since 2005 the immunization rate of Clark County has not surpassed 88.8 percent.

So, what does this mean for Clark County?

It means that people who are not immunized, which is 23.5 percent of the county according to the 2017-2018 numbers, are susceptible to viruses like the measles, which is normally well-controlled with a well-immunized population. 

The number of unvaccinated also puts people who cannot be immunized and people with compromised immune systems at risk.

RELATED: Parents with children too young for measles vaccine worry as Clark County outbreak grows

Of the people in Clark County, only 1.2 percent of people had a medical exemption, 6.3 were exempt for personal reasons and 0.3 percent were exempt for religious reasons.

RELATED: Verify: Measles outbreak in Clark County

You can still be vaccinated as an adult or teen. The Center for Disease Control and Prevention outlines what you need to know about the MMR vaccine, which protects against measles, mumps and rubella. You can read that information here.

RELATED: Verify fast facts: Measles