In The Panic Virus, journalist Seth Mnookin tracks the epidemic of fear over vaccines.

Can we just dismiss all the reports of mercury-containing vaccines causing autism?

News reports present an "on the one hand-–on the other hand" debate. In actuality, you have thousands of pieces of data on one side and, literally, no verifiable data on the other. Scientists have not identified any risk of developmental disorders; accounts of vaccines leading to autism are entirely anecdotal. Scientists don't rely on anecdotes because memory is fallible, and in retrospect things often seem to fit together more neatly than they did in the moment. Children have been diagnosed with autism after being vaccinated, but that's because almost all children get vaccinations before age two, and autism is usually diagnosed after age two.

What about the research published by Dr. Andrew Wakefield in the Lancet linking the measles/mumps/rubella vaccine with autism?

That paper was proven false. The Lancet retracted it and 10 coauthors disavowed it. Wakefield's lab received payments through a law firm that was suing vaccine makers; some of the families of children studied in the paper were involved in those lawsuits. Wakefield's data were never independently verified, but even if they had been accurate, the small sample size—12 children—means any correlation found between vaccines and autism is likely to have happened by chance.

So why doesn't the controversy die down?

The role of the media is important. Journalists don't have the training to get things right. There's been a decline in the number of science journalists, so when a scientific study comes in, editors assign it to whichever reporter is free; it's like telling your House and Homes reporter to cover the World Series. There's also intense pressure for attention-getting headlines. You're more likely to attract readers with a headline that says "Doctors are trying to harm your kids" then with a headline that says "Don't worry, everything's okay."

What's the harm if parents are cautious about vaccinations?

Because vaccines have been so effective, people think there's no downside to forgoing them: I've never seen a case of polio or measles. Unfortunately, infectious diseases are still dangerous. California this year had the most cases of whooping cough since the vaccine went into widespread use. Ten kids have died. And these weren't all children whose parents decided not to vaccinate; some were still too young to be vaccinated. You need to vaccinate a high proportion of the community to achieve "herd immunity," so contagions don't get a foothold and spread to children who can't get immunized. So when parents don't get their kids vaccinated, that endangers other kids, too.