According to the Centers for Disease Control and Prevention, a measles outbreak begins in only four cases.
Multnomah County says that while each year is different, four would be high for any county in the region.
"A case here, a few cases there, a few years without any case, and now we're talking about about 30 scattered across the border and in other countries," said Dr. Jennifer Vines of the Multnomah County Health Department.
But what makes a different outbreak? Vines says why it is unexpected, unlike the flu virus.
"The flu season is a predictable peak we have every year. We have a flu shot that you need to get every year, "Vines said.
The numbers in Clark County, however, continue to rise. There are now 36 confirmed cases of measles and 12 suspected cases.
"For this area, this is a high number," said Vines.
Only one case was identified in Multnomah County. Vines says they are preparing for the possibility of an outbreak.
But why are they preparing? Vines says it's because vaccination rates were low.
"We have seen the number of parents delaying or denying the increase in vaccines in recent years, and in addition, we have had to prepare for the outbreak response and we are using these tools now," she said.
What is an outbreak? How serious is this and can we stop it?