The measles outbreak in the United States is now the largest since the disease was declared eliminated here 19 years ago. The return of this scourge has been driven by one factor in particular: misinformation, spread by vaccine critics , that scares parents into not immunizing their children.

Along with rumors that vaccines cause autism or that the trace amounts of mercury and aluminum in them are dangerous — falsehoods that were long ago debunked — have come innuendos aimed at deeply religious parents.

Vaccines, the activists say, contain ingredients made from pigs, dogs, monkeys and aborted fetuses. Indeed, most of those assertions are based in fact . Ingredient lists published by the Centers for Disease Control and Prevention and the Institute for Vaccine Safety at Johns Hopkins show that vaccines may contain these elements (although any residual DNA is present only at the parts-per-million level).

Nonetheless, vaccination is endorsed by top Jewish and Islamic scholars, and by the Vatican. Religious authorities have meticulously studied how vaccines are made and what is in them, and still have ruled that they do not violate Jewish, Islamic or Catholic law.