SEATTLE — Measles, declared eliminated as a major public health threat in the United States almost 20 years ago, has re-emerged this winter in the Pacific Northwest and other states where parents have relatively broad leeway over whether to vaccinate their children.

Seventy-nine cases of measles have been reported by the Centers for Disease Control and Prevention since the start of this year. Fifty cases of the highly contagious disease were in Washington State.

An outbreak of measles has also occurred in the Orthodox Jewish community in Brooklyn, where 64 confirmed cases of measles were reported, mostly late last year. That outbreak began, the C.D.C. said, when a child who had not had a measles vaccination caught the virus on a visit to Israel, where a large outbreak of the disease was occurring.

But no place has been hit harder since January than Clark County, Wash., a fast-growing corner of the metropolitan area near Portland, Ore. Clark County health officials declared a medical emergency last month, and say they have seen 49 cases — most of them in children under the age of 10.

Clark County has one of the lowest vaccination rates in Washington State. About 78 percent of the kindergarten through high school population is vaccinated, according to state figures. Along with other cities mainly in the West — including Seattle, Phoenix, Salt Lake City and Houston — Portland is considered a hot spot for families that opt not to vaccinate for medical, philosophical or religious reasons.

For measles, epidemiologists generally consider the threshold for preventing public measles outbreaks to be a vaccination rate of 93 percent or higher.

“If you have a population that is unvaccinated, it’s like throwing a match into a can of gasoline,” said Dr. Alan Melnick, Clark County’s public health director. “Measles is exquisitely contagious and immunization rates have been dropping.”

Measles can cause permanent neurological damage, deafness and in relatively rare cases, death. All states allow parents to exempt children from vaccination for medical reasons, and most also allow for a religious exemption, according to the National Conference of State Legislatures.

But 17 states, including Washington, Oregon, Colorado and Texas, have gone further, allowing parents to keep their children from being vaccinated for unspecified personal or philosophical reasons. Some may be connected to a broader anti-vaccination movement, including concerns that vaccines lead to autism, an idea that has been widely debunked.

“I’m very worried that these measles epidemics are becoming a new normal,” said Dr. Peter Hotez, a co-director of the Texas Children’s Hospital Center for Vaccine Development at Baylor College of Medicine in Houston. He said that misinformation had been spread about vaccines, and that state lawmakers in some cases have allowed those claims to drive their decisions about legislation.

“The enablers are state legislators in those states, that have allowed themselves to be played,” he said.

Dr. Melnick, the Clark County official, said he had heard recently from doctors that parents have been showing up in greater numbers seeking vaccinations since the outbreak emerged. A plan is also underway, he said, for a mass clinic to give free shots.

“We’ve already had a child hospitalized; I hope it doesn’t take a death or a real serious complication like encephalitis for people to change their minds,” about vaccination, he said.

In 2018, 17 measles outbreaks were reported, mostly in pockets around the nation. In 2017, 75 cases were reported in Minnesota, in a Somali-American community with low vaccination coverage. In 2015, 147 cases across multiple states were tied to an amusement park in California, which an infected person was said to have visited, according to the C.D.C.

In Washington State’s Legislature, a bipartisan group of lawmakers is pushing to reverse the state’s rule allowing philosophical or personal exemptions to vaccination. A bill, filed in the midst of the measles outbreak, has yet to be voted on.