Health officials say 23 people have contracted measles in Oregon so far this year — the most seen since 1991.

Clackamas and Multnomah counties have seen nine cases combined in just six weeks.

People get measles while traveling and they bring it back with them.

A reluctance to immunize among some populations means Oregon and the nation are seeing a resurgence in the highly contagious disease.

Multnomah County health officer Dr. Jennifer Vines said officials are watching a new outbreak since July, and they want parents to get their kids vaccinated.

“As families get ready for back to school, remember that viruses and diseases spread in schools, and one of the best things you can put on your ‘back to school’ to-do list is to make sure your kids and your household are all up to date on vaccines,” Vines said.

Multnomah County said none of the nine recently diagnosed individuals had been immunized. All have remained at home while contagious and are in regular communication with health officials.

County officials said those people pose no risk to the public.

“Measles is a serious illness, people are miserable for a week and very contagious while sick,” Vines said. “The vaccine is safe and effective; it keeps kids healthy and in school. Fully vaccinated kids also protect their siblings, friends and teachers.”

Measles spreads through the air after an infected person coughs or sneezes. The virus can linger in the air for up to 2 hours.

According to the Centers for Disease Control and Prevention, the nation as a whole has seen some 1,200 measles cases across 30 states this year. It’s the worst outbreak in almost three decades.