Pertussis continues to spread in California, with 3,458 cases reported between January 1 and June 10: In less than half a year, the toll of the sickened already has exceeded all reported cases for 2013. State health officials have now declared a pertussis epidemic—deeply worrisome, considering the bacterium* poses a particularly dire threat to infants.

So why is the dread “whooping cough”—which had become so rare that it was almost quaint, on a rough par with infectious diseases of the microbe-haunted past such as yellow fever and plague—resurgent? There are a couple of reasons, observes Arthur Reingold, chief epidemiologist for the UC Berkeley School of Public Health and head of a federal advisory panel’s working group on pertussis.

First is the vaccine now in wide use.

Fifteen years ago, Reingold notes, the American health system switched from “whole-cell” pertussis vaccine to an “acelluar” variant. Whole-cell vaccines were exceedingly effective in providing long-term immunity, but they could induce strong reactions, including high fevers, pain at the injection site, and in some cases, seizures. “There were also some concerns at that time that they could cause permanent neurological damage,” he says. “Very thorough research has confirmed this is absolutely not the case, however.”

Acellular vaccines usually don’t cause robust reactions, and they seem to provide good immunity, continues Reingold. But there is evidence that their protective effects wane after a few years; whole-cell vaccines simply afford better immunity over the long term.

“So what we probably have is a population (inoculated with acellular vaccines) with decreasing immunity over time,” he says. “The duration of protection is shorter, but vaccinated individuals may still be only mildly symptomatic or even wholly asymptomatic if they contract the bacterium. At that point, however, they can still transmit the disease even if they aren’t showing symptoms, so unvaccinated individuals are at risk.” He adds that the efficacy of acellular vaccines might be enhanced by boosters at regular intervals, though insufficient research has been done to determine the appropriate intervals.

Reingold’s allusion to unvaccinated people points to the second problem: the large number of citizens who have opted out of vaccinations altogether. For “anti-vaxxers,” the issue often is charged with deep emotion. (This was evident when a Q&A with Reingold, published in the Summer 2014 issue of California magazine, was posted online. The article drew a huge reader response, including a protracted and heated exchange between a fervent vaccine opponent commenting under the name “Sunshine” and Sunshine’s many detractors.)

Such “vaccine hesitancy,” as Reingold diplomatically phrases it, isn’t a problem just in the United States. The developing world also is grappling with increasing opposition to vaccination programs, and the stakes there are even higher. In northern Nigeria, for example, polio is on the rise due to local fears that vaccines are part of a plot to disseminate HIV and sterilize girls and young women. And the revelation that the CIA launched a bogus hepatitis B vaccination campaign in rural Afghanistan and Pakistan as a cover for its search for Osama bin Laden didn’t exactly engender enthusiasm for legitimate inoculation programs among the locals.

“There are 200-year-old car­toons that show vac­cin­ated people turn­ing in­to cows. So this is noth­ing new. What is new is the in­ter­net.”

In the United States, the prime resistance had been concern that the mercury used as a preservative in some vaccines caused autism. Those claims, which were widely reported—some may say fanned—in the popular press have been thoroughly debunked, and The Lancet, the journal in which key research was published, has retracted it. But the fears remain, and have indeed expanded, particularly among a certain cohort. Anti-vaxxers span the political spectrum, from far left to hard core right, many sharing a deep distrust of officialdom and a penchant for conspiracy theories. Still, Reingold says, their obsession is hardly a recent development; vaccine resistance has been around as long as vaccines.

“The first inoculations were carried out in the 1790s against smallpox,” he says. “Cowpox, a related virus that conferred some immunity, was used. And there was an immediate strong and negative reaction. There are 200-year-old cartoons that show vaccinated people turning into cows. So this is nothing new. What is new is the internet. It just makes it much easier to promulgate misinformation as ‘scientific research.’ “

Reingold is measured, even mild, when addressing anti-vaxxers. His restraint could be considered remarkable, considering that about five years ago, he was menaced by a disturbed student who was apparently set off by Reingold’s contention that vaccines should not be linked to autism. The man, who Reingold says had a history of of “crazy episodes,” showed up his office one day with a hammer and smashed everything in the place. (Fortunately, the epidemiologist was in Geneva at that time, and Reingold said he decided not to press charges after the student signed a legal document vowing to stay away from him.)

The vast majority of vaccine resisters, of course, are well-intended and believe that they are acting in the best interests of their own children. Many emphasize that vaccines can trigger serious side effects, others accuse the government of being in cahoots with the pharmaceutical industry, and still others claim, without any credible scientific support, that vaccines cause the disease they are intended to protect against. “I am not wrong!” Sunshine wrote. “I’ve done my children a high service by not following the sheep and you know what, many parents are not following the sheep, or in this case I should say wolves….The growing polarization regarding vaccines is unmistakable.”

The result? Research has concluded that the few states (including California) that make it easy for parents to opt out of vaccinating their children experience a far higher incidence of pertussis, and that clusters of non-medical vaccine exemptions may have contributed to an outbreak of pertussis in California in 2010.

So how can the science and medical communities best address the anti-vax furor? The U.S. Supreme Court has ruled that people may opt out of vaccinations for religious, medical, or other compelling reasons, observes Reingold, so the courts offer little scope.

“But vaccinations can be made a requirement for specific programs,” he says. “So if you want particular services, or you want to attend a school or engage in a particular activity, you could be compelled to show proof of immunization.”

Ultimately, however, scientists say education may be the best approach.

“We need to start at grammar school,” Reingold says. “For whatever reason, there is a distrust of science in some segments of the population. We need to counter that distrust with a comprehensive program that explains the benefits of proven, science-based medical policies.” And he adds that people who favor vaccinations—and have compelling stories to support their positions—must also be willing to engage on the issue.

Reingold is on the federal Centers for Disease Control’s Advisory Committee on Immunization Practices, chairing the committee’s working group on pertussis. “We report directly to the (U.S.) Department of Health and Human Services. We hold three meetings each year that are open to the public. Anyone can say anything they want,” he says. “In the past, these meetings were heavily attended by people opposed to vaccination, but recently, hardly any vaccination opponents have appeared. On the other hand, we have had a growing response from people who have had kids who have been injured or even killed by diseases that are preventable by vaccinations. Their stories are incredibly moving. More than that, I think vaccination opponents have been shamed. They don’t want to show up.”

Our Summer 2014 issue features a Q&A with Berkeley’s head epidemiologist about the return of pertussis. Read more »

*This reflects a correction from an earlier version of the story.