According to the Centers for Disease Control and Prevention, a measles outbreak begins in just four cases.
The county of Multnomah says that every year is different, four would be elevated for any county of the zone.
"A case here, a couple of cases, some years without cases, and now it's about 30 extended by the border and other counties," said Dr. Jennifer Vines of the Department of Health of the County of Multnomah.
But what makes an outbreak different? Vines says it's unexpected, unlike the flu virus.
"The flu season is a predictable increase we get every year. We have a flu vaccine that must be obtained each year," Vines said.
However, Clark County numbers continue to increase. There are currently 36 confirmed measles cases and 12 suspects.
"For this area, this is a great number," Vines said.
Only one case has been detected in Multnomah County. Vines says that they have been prepared for the possibility of an outbreak.
But why are you prepared? Vines says it's because they've reduced the vaccination rates.
"We have seen that the number of parents that delay or refuse vaccines increase in recent years, so we have also had to prepare for the outbreak response and now we are using these tools," he said.
What is an outbreak? How serious is this and can we stop it?