It's always dismaying when seemingly rational, intelligent people make incomprehensible decisions.

Such is the case when parents ignore reams of scientific data, demonstrable real-world evidence and expert advice by choosing to forgo standard immunizations for their children.

What's particularly frustrating is that many of them have turned against a proven public health benefit because they've been suckered into believing the thoroughly discredited accounts linking vaccines to various ills, including autism and sudden infant death syndrome.

In recent years, the growing number of people who have bought into these widely circulated myths have prompted health officials to worry about a resurgence of horrible diseases that were once considered conquered.

Now, as parents prepare their children for a new school year and are asked to provide immunization records to their schools, it's worth taking a moment to understand the critical importance of this issue.

Consider that in communities throughout the United States, immunization rates have steadily fallen over the past several years. In the 2013-14 school year in California, 90% of incoming kindergartners had all the required vaccinations, a percentage point lower than just four years earlier, according to the California Department of Public Health.

Meanwhile, the ratio of parents opting out through the so-called "personal belief" exemption has continued to rise, reaching 3.15% in the last school year.

In Orange County, 88.7% of incoming kindergarten students had a full set of immunizations last year, while the personal exemption rate was 3.65%.

Those figures might not raise alarm bells, but they should. Extremely high immunization rates are needed to bestow what's known as a "herd effect" that protects against outbreaks. The effect is now in jeopardy in many places.

In 2010, California experienced the largest outbreak of pertussis in 60 years, with 9,000 infected, resulting in the deaths of 10 infants. This year could be equally bad or worse. As of July 11, 5,393 cases were reported statewide, and three infants had died of the disease.

Health officials said that the reemergence of pertussis, also known as whooping cough, is likely partly due to the fact that protection given by the vaccine wanes over time, and some parents might be too casual about keeping their kids current with booster shots. They also blamed the increasing number of people choosing not to immunize their children at all.

Pertussis, by the way, is a nasty, highly contagious bacterial infection that causes coughing fits and difficulty breathing. Babies are particularly susceptible, prompting officials to urge pregnant women to receive the vaccine to extend coverage to newborns.

Other diseases, once considered nearly eradicated in the United States thanks to vaccinations, are also staging a comeback. Measles cases so far this year have reached the highest number in 20 years, according to the Centers for Disease Control and Prevention. Mumps are also on the rise.

And in a dire report last May, the World Health Organization warned that outbreaks of polio, a viral disease that can cripple or kill those infected, have surfaced in pockets throughout Asia, the Middle East and Africa. The virus could be imported to the U.S. — indeed, it's just a plane ride away — posing a danger to those who haven't been fully vaccinated, babies too young to be immunized, and those who can't receive vaccinations because they have compromised immune systems.

In response to the growing threat, California enacted a law this year making it more difficult for parents to obtain a personal belief exemption for required immunizations for their children.

But it likely won't deter those who are determined to let their irrational fears and distrust of medical consensus overcome common sense. The law merely requires parents who wish to exclude their children from immunization requirements to submit a signed form showing they received information about the risks and benefits of vaccinations from a health-care professional, who must also sign the form.

Dr. Behnoosh Afghani, a UC Irvine professor and pediatric infectious disease specialist, said she encounters many parents who express reservations about vaccinations, for some reason particularly in middle-class communities in south Orange County. Often these parents have read erroneous information posted on social media sites, or they know a relative of a child diagnosed with autism who blames the disorder on immunizations.

"It's very hard for me to convince these people," she said. "A fear has been created about vaccines."

But if not enough people are immunized, there will certainly be outbreaks, she said.

Imagine a high school with 1,000 students, and 50 of those students haven't been vaccinated against the measles virus, Afghani said.

If their community is exposed to the easily transmitted disease, those 50 students will surely become infected, she added.

People might be tempted to dismiss the seriousness of that possibility, but that would be a mistake, she said. The measles virus can lead to high fevers, hospitalization, pneumonia and encephalitis, which can in turn result in long-lasting neurological problems. One or two deaths could occur.

"We need more good information to get to people" about the importance of immunizations, Afghani said.

So spread the word. The effort to nearly wipe out some terrible diseases through high immunization rates has been one of the major public health success stories of modern times. We must overcome the ignorance and misplaced fear that risk turning back the clock on that achievement.

PATRICE APODACA is a former Newport-Mesa public school parent and former Los Angeles Times staff writer. She lives in Newport Beach.