In the churning over the refusal of some parents to immunize their children against certain diseases, a venerable Latin phrase may prove useful: Post hoc, ergo propter hoc. It means, “After this, therefore because of this.” In plainer language: Event B follows Event A, so B must be the direct result of A. It is a classic fallacy in logic.

It is also a trap into which many Americans have fallen. That is the consensus among health professionals trying to contain recent spurts of infectious diseases that they had believed were forever in the country’s rearview mirror. They worry that too many people are not getting their children vaccinated, out of a conviction that inoculations are risky.

Some parents feel certain that vaccines can lead to autism, if only because there have been instances when a child got a shot and then became autistic. Post hoc, ergo propter hoc. Making that connection between the two events, most health experts say, is as fallacious in the world of medicine as it is in the field of logic.

An outbreak of measles several weeks ago at Disneyland in Southern California focused minds and deepened concerns. It was as if the amusement park had become the tragic kingdom. Dozens of measles cases have spread across California. Arizona and other nearby states reported their own eruptions of this nasty illness, which officialdom had pronounced essentially eradicated in this country as recently as 2000.

But it is back. In 2014, there were 644 cases in 27 states, according to the Centers for Disease Control and Prevention. Should the pace set in January continue, the numbers could go still higher in 2015. While no one is known to have died in the new outbreaks, the lethal possibilities cannot be shrugged off. If the past is a guide, one or two of every 1,000 infected people will not survive.

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Jeffrey R. Ungvary President