Someone with measles was at Concourse D of the Portland International Airport on Jan. 7, the county’s public health department advised. An infected person attended a Portland Trail Blazers home game Jan. 11.

At the beginning of last week, there were only a handful of confirmed cases. On Friday, the day the emergency was declared, there were 19. By Sunday, that number had grown to 21. The latest update came Tuesday, when county officials said they had confirmed 23 cases and were investigating two more suspected cases. The vast majority of those who have fallen ill had not been immunized.

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The outbreak makes concrete the fear of pediatric epidemiologists that a citadel of the movement against compulsory vaccination could be susceptible to the rapid spread of a potentially deadly disease.

“It’s alarming,” Douglas J. Opel, a pediatrician at Seattle Children’s Hospital, said in an interview with The Washington Post. “Any time we have an outbreak of a disease that we have a safe and effective vaccine against, it should raise a red flag.”

State data shows that 7.9 percent of children in Clark County were exempt in the 2017-2018 school year from vaccines required for kindergarten entry, which includes the two-dose course for measles that the Centers for Disease Control and Prevention says is 97 percent effective. Only 1.2 percent of the children had a medical dispensation, meaning that nearly 7 percent were not immunized for personal or religious reasons. Nationally, about 2 percent of children went without required immunizations for nonmedical reasons.

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The high rate of nonmedical exemption for vaccines is what makes the Portland area, which sits across the Columbia River from Clark County, a “hotspot” for outbreaks, according to Peter J. Hotez, a professor of pediatrics and dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston.

“This is something I’ve predicted for a while now,” he said of the public health emergency in Clark County. “It’s really awful and really tragic and totally preventable.”

Of the confirmed cases, 18 patients are between the ages of 1 to 10 years old. Twenty of the infected individuals had not been immunized against measles, and the vaccination history of the other three remained unverified. One person was hospitalized.

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Experts advised that the outbreak could still be in its infancy. The incubation period of the virus averages two weeks, and it can be spread four days before a rash makes its onset obvious.

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Because measles is among the most highly contagious of all infectious diseases, it is bound to flare up in areas with low vaccination rates, Hotez said. He tracked this effect in a paper last year in the Public Library of Science, linking the number of philosophical exemptions, which has climbed since 2009 in 12 of the 18 states that allow them, to increasing outbreaks.

The problem is especially pronounced, the paper found, in more than a dozen “hotspot metropolitan areas,” including Portland and Seattle in the Northwest, Phoenix in the Southwest and Detroit in the Midwest.

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Public health experts are sounding alarms about the geographical clustering of people who refuse to immunize themselves, which creates vulnerabilities despite the overall high rate of vaccination. In November, Asheville, N.C., another stronghold of the anti-vaccination movement, succumbed to the state’s worst chickenpox outbreak since a vaccine for the infection became available more than two decades ago.

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“Portland is a total train wreck when it comes to vaccine rates,” Hotez said in an interview with The Post.

Opposition to compulsory vaccination in the Pacific Northwest dates to the Progressive Era and continues despite major medical breakthroughs. The modern anti-vaccination movement — built on debunked research published in 1998 that associated the vaccine for measles, mumps and rubella, known as the MMR vaccine, with autism — is not exclusive to one side of the political divide, survey data suggests; it tends to find its most fervent supporters at both extremes.

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Measles is a dire price to pay for leniency about vaccination, Hotez cautioned, calling the illness “one of the most serious infectious diseases known to humankind.” After smallpox was eradicated in 1980, measles became the leading killer of children globally, he said.

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In 2000, public health officials declared measles eliminated in the United States because more than a year had gone by without continuous transmission of the disease. But recent outbreaks supply evidence of dangerous backsliding in containment of the virus, Hotez said, blaming the anti-vaccination movement. “This is a self-inflicted wound,” he said.

In 2015, a woman in northwestern Washington died of pneumonia after contracting measles. It was the first U.S. death from the virus since 2003.

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Last year saw the second highest number of reported cases of measles since 2000, according to the CDC. A total of 349 cases were confirmed across 26 states and the District of Columbia, only surpassed by the 667 cases in 2014. Orthodox Jewish communities were at the center of several outbreaks last year in New York and New Jersey, after unvaccinated travelers returned with the virus from Israel, which was battling an outbreak. The year before, Minnesota reported 75 cases of measles, mostly in a Somali community where the discredited theory blaming autism on the MMR vaccine had taken hold.

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Because measles is still endemic in parts of the world, said Opel, “periodic introduction by people traveling is what’s causing frequent outbreaks here.”

A high level of protection is required to prevent transfer of the highly contagious virus, he added. Somewhere between 92 and 94 percent of the population must be immunized. Clark County is already below that level, he observed, “before you factor in other things like people just missing their appointments.”

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The county’s health department emphasized how easily the virus can spread, remaining for as long as two hours in the air of a room where an infected person has been.

“If other people breathe the contaminated air or touch a contaminated surface, then touch their eyes, noses or mouths, they can become infected,” the county warned in a statement Tuesday. “Measles is so contagious that if one person has it, 90 percent of the people close to that person who are not immune will also become infected.”

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While the illness often begins with cold-like symptoms and a rash, doctors emphasized that many infected people suffer from additional complications, including pneumonia and, in more serious cases, inflammation of the brain known as encephalitis and even seizures.

“It’s not a benign illness,” Opel said.