A public health emergency has been declared in Clark County, Washington, as a result of an ongoing measles outbreak. According to the latest update from the county's Department of Health, 23 cases of measles have been confirmed and health officials are investigating two more suspected cases.

The majority of cases are in children aged 1 to 10. Four cases are in children and teens aged 11 to 18, and one person aged 19 to 29 has been infected.

Measles is an extremely contagious infection that is serious for small children but is easily preventable by vaccine. Symptoms, which include cough, runny nose, inflamed eyes, sore throat, fever, and a red, blotchy skin rash, usually develop 10 to 12 days after exposure and last 7 to 10 days.

Get Breaking News Delivered to Your Inbox

The virus lives in the nose and throat mucus of an infected person and can spread to others through coughing and sneezing. Additionally, the measles virus can live for up to two hours in an airspace where the infected person coughed or sneezed. Then if others breathe the contaminated air or touch the infected surface and touch their eyes, noses, or mouths, they can become infected.

Measles is so contagious, in fact, that if one person has it, 90 percent of the people close to that person who are not immune will also become infected, according to the Centers for Disease Control and Prevention.

Dr. Jonathan Mou at Adventist Health in Portland emphasizes that while the virus is highly contagious, it's also one of the most preventable.

"Once it sets into a community, the likelihood of it getting beyond that community or more widespread within that community is high, especially when vaccination rates are low," Mou told CBS affiliate KOIN-TV.

Mou said some people aren't able to get the vaccine due to immune system issues or an allergic reaction, so if you can get the shot, it won't just help you, it will provide "herd immunity" for others.

Portland area an anti-vax "hotspot"

While some children cannot be fully vaccinated for medical reasons, others skip the shots because of their parents' religious or philosophical beliefs. A major reason behind choosing not to vaccinate children is the false and disproven claim that vaccines were linked to autism.

Eighteen states in the U.S. currently permit non-medical exemptions from childhood vaccinations due to philosophical beliefs. An analysis published last June in the journal PLOS Medicine found an increase in the number of children enrolling in kindergarten with a non-medical exemption in 12 of these states.

The study also identified 15 metropolitan clusters or "hotspots" of higher rates of vaccination exemptions. The Portland, Oregon, area, just across the Columbia River from Clark County, is one of those hotspots.

"As larger unvaccinated populations grow, particularly in highly mobile cities, the potential for vaccine-preventable disease outbreaks grows," study authors Peter Hotez and Melissa Nolan said in a joint statement. "Measles outbreaks are of particular concern because measles is so highly transmissible and is associated with high morbidities, leading to hospitalization and sometimes permanent neurological injury or even death."

Clark County health officials are urging anyone who has been exposed to the measles virus and believes they have symptoms to call their health care provider before visiting the medical office to make a plan that avoids exposing others in the waiting room. People who believe they have symptoms of measles should not go directly to doctor's offices, urgent care centers, or emergency rooms (unless experiencing a medical emergency) without calling in advance.