This article provides a review of the science of HIV/AIDS for nonspecialist stakeholders such as lawyers, regulators, and policy analysts. The purpose of the article is twofold: it (1) aims to communicate the biomedical principles that underlie the pandemic in a rigorous yet accessible format and (2) shows that an understanding of the interaction between a legal rule and medical science is essential if the intended meaning, purpose, and policy rationale of the rule are to be preserved. It illustrates the interaction between law and science by analyzing the application of a doctrine in medical malpractice to litigation involving HIV/AIDS. It then draws parallels to other emerging diseases and shows how the analysis can be generalized and extended to legal issues involving any novel disease.