The number of confirmed cases of measles near Portland has increased from 25 to 30 on Friday and there are nine others suspected of being ill.
The outbreak of measles is boosted by a lower than usual incidence of vaccination in what has been identified as the place of breeding of vaccine opponents in the United States.
Health authorities in southwest Washington state beyond the Columbia River said humans could be exposed to the dangerous disease in more than thirty places, including at Portland International Airport. Trail Blazers, Amazon Locker location and stores such as Ikea and Costco.
Twenty-six of the confirmed patients have not been vaccinated against measles, and the vaccination status of four others who have been infected is unknown. One person was hospitalized.
Most cases include children under 10 years of age. One adult gets infected and the rest is a teenager.
Washington Governor Jay Inslee announced state emergency health on Friday, and authorities in neighboring states of Idaho and Oregon also issue residents' alerts.
The number of cases "creates a serious risk to public health, which can quickly spread to other countries," Inslee said.
Clark County, which includes Vancouver's residential area in Washington, has 78% of measles vaccinations, which is well below 92% or 94%, which is required for so-called "collective immunity," said Marisa Armstrong, spokeswoman for Ministry of Health.
Collective immunity is achieved when vaccinated individuals are protected from infection because almost all of them are vaccinated and become immunized against the disease.
The measles virus, which spreads through coughing or sneezing, can remain in the air for up to two hours in an isolated space. Nine percent of people exposed to measles who have not been vaccinated will become ill.