In a feat that would have been unimaginable a few decades ago, the anti-vaccine movement has managed to breathe life into nearly vanquished childhood diseases.

It took all the ingenuity and know-how we are capable of to find safe, effective ways to dramatically diminish diseases like measles and whooping cough in the developed world; it took all the hysteria and willful ignorance we are capable of to give them a boost. A developer of the measles vaccine, Dr. Samuel Katz, says the question “is not whether we shall see a world without measles, but when.”

Not if Jenny McCarthy has anything to say about it. The former Playboy model and current co-host of “The View” is a leading light of the anti-vaccine movement. She has a boy with autismlike symptoms that she is convinced were caused by the vaccine for measles, mumps, and rubella (MMR). You can credit her passion for her child, sympathize with her heartbreak — and still cringe at her wholly irrational cause.

It is only natural that parents who see their young autistic children slip away at about the same time they receive vaccinations make the mistake of confusing correlation and causation. And it is only human to want to believe that a tragedy is a morality tale with readily identifiable villains, in this case the drug industry and the medical establishment. None of this makes the so-called anti-vaxxers any less wrong, or doggedly impervious to evidence.

No amount of discrediting makes a difference. One theory was that a preservative in children’s vaccines called thimerosal was causing autism. But the U.S. removed thimerosal from most childhood vaccines in 2001. If the theory had been sound, this should have reduced cases of autism. It didn’t. Cases have continued to rise, and the same held true in Canada and Denmark after eliminating thimerosal in the 1990s.

Another theory, latched onto by Jenny McCarthy, is that the MMR vaccine in particular causes autism. Dr. Andrew Wakefield publicized this supposed link in a famous article in the British medical journal The Lancet. It has since been thoroughly debunked. The Lancet retracted Wakefield’s paper, and the British Medical Journal reported that he “falsified data.” He had his medical license revoked. All of which should have been enough to give the anti-vaxxers pause.

Nonetheless, they fight on. In an interview with the Fox Business Network the other day, former MTV star Kristin Cavallari plugged the anti-vaccination cause, citing “books” and “studies.”

Most parents don’t listen. Only 1.8 percent of kindergartners get exempted from vaccinations, according to NBC News. But the number is higher in some states. In Oregon, the rate is 6.4 percent, with some counties hitting double digits. In California, Marin County has an exemption rate of nearly 8 percent. The more kids go unvaccinated, the greater the chance that diseases can get a foothold.

They usually are imported from abroad, but the absence of vaccination is a boon to their spread. A study in the journal Pediatrics found that the 2010 whooping-cough outbreak in California — when the state had the highest number of cases since 1947 — hit hardest in areas with high levels of nonvaccination. In 2013, measles cases tripled nationwide. Outbreaks were centered in religious communities in Brooklyn, N.Y., Texas and North Carolina that had resisted vaccination. New York City has another small outbreak right now.

In the panic created by the Wakefield article, England saw MMR vaccination rates fall to 80 percent in 2004 and Wales to 78 percent. In 2012, England and Wales had the highest number of measles cases in 18 years.

These are dangerous illnesses, and the victims of an outbreak are often infants too small to have yet received vaccinations. Jenny McCarthy styles herself a “mother warrior.” If so, the kids sickened in the fallout from reduced vaccinations are the victims of friendly fire. Nothing good can come from undoing one of the miracles of medical progress.