The outbreak marks a sharp reversal for a disease that had been considered effectively eliminated in the United States for decades. Measles is among the most contagious viruses known, capable of spreading through a room even after an infected person has left, and it can cause severe complications including pneumonia, brain swelling and death, particularly in young children and immunocompromised people.

The pattern reflects what public health experts describe as a growing "vaccine divide" — a widening gap between communities with high immunization rates and those where coverage has slipped below the threshold needed to prevent outbreaks. Once vaccination rates fall under roughly 95 percent, herd immunity weakens and outbreaks become far more likely.

The decline is not uniform. Public health data has consistently shown that pockets of low vaccination tend to cluster geographically, meaning some schools and communities face far greater risk than others. That uneven landscape has made Oregon vulnerable even as statewide averages remain relatively high.

State health officials have urged families to check their children's immunization records and to catch up on missed doses of the measles, mumps and rubella vaccine. Two doses of MMR are about 97 percent effective at preventing measles.

The resurgence carries particular weight for Oregon families with infants too young to be vaccinated and for people with weakened immune systems who cannot receive live vaccines and depend on the immunity of those around them.

Public health advocates say the trend underscores the importance of trusted, accessible vaccination services and clear communication from local health authorities, especially in communities where confidence in vaccines has eroded.