An outbreak of measles four years ago at Disneyland focused attention on a growing health menace — the refusal of parents to vaccinate their children. The threat has gone international. The World Health Organization has just named the anti-vaccination movement among the 10 biggest global health crises.

Italy is ground zero, thanks to a law pushed by the far-right 5-Star Movement that ended compulsory vaccinations for children in public schools. Matteo Salvini, leader of its coalition partner, the League party, called mandatory vaccinations “useless and in many cases dangerous.”

The anti-vaxxer crusade has a diverse membership. In addition to traditional right-wingers and radical libertarians who say the decision to not immunize their children should be a matter of personal liberty, it includes rich progressives who view vaccinations as unhealthy. (Far more students in California’s well-to-do Capistrano Unified School District were found to be unvaccinated than in Santa Ana, its poorer neighbor.)

The “vaccine-hesitant” — the WHO’s politer term — often wave ignorant junk-science claims that vaccines can cause autism. This dangerous lie gained traction in a 1998 article published in the prestigious British journal The Lancet. It turned out that lawyers suing vaccine-makers were funding the author, Andrew Wakefield. Britain subsequently stripped Wakefield of the right to practice medicine.

Donald Trump promoted the falsehood, linking vaccines and autism during a primary debate. The Autistic Self Advocacy Network condemned the remark.

Measles cases in Europe rose to 60,000 last year, including 72 deaths, more than double the number in 2017. Consider that this disease was once close to being eradicated.

In this country, 18 states still allow “nonmedical exemptions” for vaccinating children based on a philosophical belief. Requests for such exemptions are rising in 12 of them.

New York state is seeing its most troubling measles outbreak in decades. Almost all the cases occur among ultra-Orthodox Jews, whose insular communities have been ripe for anti-vaccine propagandists. William Handler, an Orthodox rabbi in Brooklyn, told Vox that parents who “placate the gods of vaccination” are engaging in “child sacrifice.” That’s not far off from a League party official’s nutty labeling of state-funded vaccinations as “free genocide.”

Once vaccination levels fall below 95 percent, epidemiologists explain, there aren’t enough people to hold the disease in check. And measles is highly contagious. The virus floats in the air and can live on surfaces for hours.

Of the 52 measles cases linked to the Disneyland outbreak, six of the afflicted had been vaccinated. Health officials note that for a few people, vaccinations don’t produce a strong enough antibody response to stop the disease entirely. But these patients tend to have far milder versions of measles and are less likely to pass it on to others.

Measles is especially dangerous to pregnant women and the very young. Some of the victims at Disneyland were children not old enough to be vaccinated.

The WHO worries that the anti-vaccination movement in the U.S. and Europe may invade less rich countries, such as Brazil, India, China, Indonesia and Nigeria. Then we could have enormous populations spreading an epidemic.

That’s why the WHO put “vaccine hesitancy” right up there with an Ebola outbreak and influenza pandemic as being among the 10 biggest threats to world health in 2019.

California passed a law in 2016 mandating vaccinations. Anti-vaxxers challenged it and lost. In explaining its decision, the state appeals court quoted from a 1944 U.S. Supreme Court ruling: “The right to practice religion freely does not include the liberty to expose the community … to ill health or death.” Clearly, the states must lead the charge in fighting the irresponsible and scientifically illiterate opposition to vaccination.

Follow Froma Harrop on Twitter @FromaHarrop. She can be reached at fharrop@gmail.com.