The governor of Washington state declared a state of emergency Friday over a measles outbreak that has sickened dozens of people in a county with one of the state's lowest vaccination rates.

Gov. Jay Inslee said in a statement that the outbreak in Clark County "creates an extreme public health risk" that could spread throughout the state. Under Inslee's declaration, state agencies can use all available resources to help affected areas and additional medical resources can be requested from other states.

Clark County Public Health has confirmed 30 measles cases since January 1 and identified another nine suspected cases. Twenty-six of the confirmed cases were people who were not immunized for measles, the agency said. Most of the confirmed cases — 21 — were with children between 1 and 10 years old. Eight cases involved people 11 to 18 years old, and one case was someone 19 to 29.

Get Breaking News Delivered to Your Inbox

The county released a list of public places where infected children visited, which included several schools and churches, the Portland, Oregon airport and a Costco.

One case has been confirmed in King County, where Seattle is located. It involved a man in his 50s who was hospitalized.

Clark County — which is in the southwest part of Washington and close to Portland — has a lower vaccination rate than nearly any other county in the state. Only 77.4 percent of all public students there complete their vaccinations, according to state records cited by the Oregonian.

Measles is a highly-contagious and potentially deadly infection that is most common for small children, although it is easily preventable through vaccination. Symptoms include cough, runny nose, sore throat, fever and blotchy skin rashes. The disease is so contagious that 90 percent of people close to someone who is not immune will be infected, according to the Centers for Disease Control and Prevention.

The source of the outbreak affecting Washington has not been determined.