I am a believer in the power of facts. I was raised by a scientist; I used to be one of those kids who read encylopedias for fun. (Stop snickering.) When confronted with some voguish piece of populist lore, my basic instinct is to hold it at arm’s length, poke at it with a ruler (graduated in hundredths of an inch), and see if I can deflate it.  (I’ve taught myself not to do this when people promote their pet theories at cocktail parties, because it doesn’t get me invited back.)

Which brings me to the subject of vaccines. Like every thinking (or at least Website-reading) parent right now, I have been exposed to a lot of pro- and anti-vaccine opinion, and even the fact of that infuriates me. This is a subject requiring real information, and (as this excellent story in Wired, by Amy Wallace, says), “Because a massive research effort has yet to reveal the precise causes of autism, pseudo-science has stepped aggressively into the void.” The chief stepper there is Jenny McCarthy, a woman who thinks smallpox vaccines are dangerous but a pound or so of Dow Corning silicone gel, parked in the intramammary fold of an 18-year-old, is perfectly acceptable. Some toxins—that’s the nonspecific word these people use, “toxins,” because it means anything they don’t like—are more acceptable than others.

The most fundamental way reason vaccines work is that we all agree to use them, because of what’s called “herd immunity,” and that’s especially important because not every vaccine takes. Occasionally, a dose doesn’t do its job. If my kid is one of those whose vaccine didn’t take, the fact that you allowed your kid to be a carrier puts him at risk.

Let the number of non-vaccinated members of the population creep up a little more, and herd immunity evaporates. If 20 percent of us refuse vaccines, the wildfire nature of infectious disease returns, and we are practically back in the Middle Ages. Which is, of course, the basic problem: Short memories. Until two generations ago, everyone knew a family who’d lost a couple of kids, or seen them badly damaged, by basic childhood diseases. Mumps, scarlet fever, diphtheria. Blindness, deafness, disfigurement.

This week my little guy got his flu shot. (He was a trouper.) And yet, even from my rock-solid belief system … that awful stuff creeps into my head. Mercury. Autism. Big Pharma’s perfidies. The  study, ten years down the road, that finds a benign ingredient to be anything but. This is how they work, the zealots, by planting that fearful voice in the back of your head.  And they’re able to make some connection it to the real reason you’d skip the needle, the one that doesn’t have any science attached at all, just a basic humanity: It’s a little bit of a relief to skip anything that involves a sharp piece of steel in your sweet baby’s flesh. I cringed the first time I saw it; I cringed again when it went in. And if I’d been a little more ignorant, I might’ve listened to Jenny and her crowd for just a bit longer. That may be the fearmongers’ best weapon.