For more than a decade, families across the country have been warring with the medical establishment over their claims that routine childhood vaccines are responsible for the nation’s apparent epidemic of autism. In an extraordinary proceeding that begins today, the battle will move from the ivory tower to the courts.

Nearly 5,000 families will seek to convince a special “vaccine court” in Washington that the vaccines can cause healthy and outgoing children to withdraw into uncommunicative, autistic shells – even though a large body of evidence and expert opinion has found no link. The court has never heard a case of such magnitude.

The shift from laboratory to courtroom means the outcome will hinge not on scientific standards of evidence but on a legal standard of plausibility – what one lawyer for the families called “50 percent and a feather.” That may make it easier for the plaintiffs to sway the panel of three “special masters,” which is why the decision could not only change the lives of thousands of American families but also have a profound effect on the decisions of parents around the world about whether to vaccinate their children.

A victory by the plaintiffs, public health officials say, could increase the number of children who are not given vaccines and fall sick or die from the diseases they prevent.