MaryJo Perry raises animals on her property outside of Jackson, Miss. , and uses ivermectin to treat her cattle. To her, the drug is as familiar, safe, cheap and effective as vitamins: "We've been using it on the farm for 40 years.

Longtime allies of Robert F. Kennedy Jr. , the nation's health secretary, have launched a new effort to repeal laws that for decades have required children to be vaccinated against measles, polio and other diseases before they enter day care or kindergarten.