A rash followed by itchy, fluid-filled blisters all over a child’s body, usually tells parents their child has chickenpox. The illness was once a standard part of growing up—and many have the scars to prove it—but since a vaccination became common practice, fewer kids contract the virus. In 1995, the Advisory Committee on Immunization Practices, a group of medical and public health experts at the Center for Disease Control and Prevention (CDC) who recommend which vaccines should be used routinely…