It all started with bad science. The now-debunked theory that autism is caused by the common immunizations nearly all children receive beginning in infancy began with a fabricated piece of research, a 1998 study published—and later retracted—in the journal Lancet. In 2010, Great Britain stripped Andrew Wakefield, the lead author of the study, of his medical license. An investigation had deemed his research an elaborate fraud.

But in those dozen years, fear of lifesaving immunizations took hold of millions of parents. Jenny McCarthy—former Playmate of the year, model, actress, and soon-to-be cohost of the television show The View—fueled parental fears. She built a movement around the flawed theory. McCarthy, who has an autistic son, wrote Louder Than Words: A Mother's Journey in Healing Autism, correlating the increase in childhood vaccinations with the rise in autism worldwide.

"She is absolutely entitled to her opinion, but to say that it's fact when it's not fact is just wrong," says Glenn Braunstein, vice president of clinical innovation at Cedars-Sinai Medical Center in Los Angeles. "It's one step down from yelling 'fire' in a crowded theater when there is no fire. It's fear-mongering."

But the movement snowballed. Congress held hearings. More than 5,000 people petitioned a newly formed Vaccine Injury Compensation Program, charging that vaccines injured their children. (Courts eventually found no proven link between vaccines and autism.) Parents began saying no to immunizations, and the percentage of parents who delayed or refused vaccinations rose from 22 percent in 2003 to nearly 40 percent in 2008. For the first time in decades, the U.S. saw outbreaks of diseases like measles, mumps, and whooping cough.

Completely Discredited

The original research began to be discredited as early as 1999, when two studies commissioned by the U.K. Department of Health found no evidence that immunizations were associated with autism. In 2001, a panel of 15 experts from the Institute of Medicine, which advises Congress, found no connection between the measles, mumps, and rubella (MMR) vaccine and autism. In 2004, a comprehensive review by the Institute of Medicine found no causal relationship between vaccines and autism.

But it would take almost another decade for the furor to even begin to die down. A study this year in The Journal of Pediatrics may at last put the final nail in the coffin of the discredited research. In April, researchers published a study that looked at nearly 1,000 children and concluded that exposure to vaccines during the first two years of life was not associated with an increased risk of developing autism.

Maybe, just maybe, Jenny McCarthy won't even mention autism and vaccines from her new perch on The View. That's the hope of Paul Offit, chief of the Division of Infectious Diseases at the Children's Hospital of Philadelphia. "In a more rational world, this discussion would be un-reopenable," Offit says. "The answerable questions have all been answered." It's not the vaccine, or anything in the vaccine. It's not the number or timing of vaccinations. Scientifically, he says, we know that.

So what is causing an increase in autism? We don't know for sure, says Offit, but the best data are genetic, involving several genes required for brain development that may generate abnormalities even in the womb. Some researchers have found a connection between older fathers and an increased risk of autism in their children. Or the increase could be due to more awareness of autism and a broader definition of the disorder.