Immunization helps prevent acquisition of diseases, many of which were once common in the United States. Immunity is generally achieved by administering live or dead pathogens or their components. Vaccines used for active immunization consist of live (attenuated) organisms, killed whole or split organisms, microbial components, or inactivated toxins (toxoids). Some cases require the administration of antibody-containing gamma globulin injections. According to the new Centers for Disease Control and Prevention Guideline on infection control, dental practices must develop a written, comprehensive policy on immunizing workers and refer personnel to qualified professionals for evaluation and, when necessary, for all appropriate immunizations.