Awesome video by AsapSCIENCE on Plan B. The video does a great job at explaining in very simple terms how the menstrual cycle functions, how women become pregnant and, most importantly, how Plan B does not prevent implantation of an egg to the uterus or abort a pregnancy.
Public Health is the science of protecting and improving the health of communities through education, promotion of healthy lifestyles, and research for disease and injury prevention.
(What is Public Health? Association of Schools of Public Health )