Reactive devaluation is a cognitive bias in which proposals, ideas, or arguments are rejected, minimized, or underestimated because they originate from a source perceived as antagonistic, unreliable, or contrary, regardless of their validity. This chapter illustrates the phenomenon through the story of Ignaz Semmelweis and his discovery of handwashing as a strategy to prevent puerperal fever. Despite the dramatic reduction in mortality in the ward he managed, his proposal was rejected by the Viennese medical elite, as it required acknowledging that physicians themselves were responsible for maternal deaths and because the theory of “cadaveric particles” contradicted prevailing paradigms. The adverse reaction toward Semmelweis, perceived as defiant and accusatory, triggered a systematic devaluation of his evidence, even after it was formally presented. The chapter reviews the historical origins of the bias, from the studies of Lee Ross and Constance Stillinger in the 1980s, and analyzes its personal and collective effects, including the creation of prejudice, the weakening of cooperation, the inability to resolve conflicts, and professional isolation. It also examines its impact on contemporary medical practice, where it can hinder decision-making in multidisciplinary boards, affect collaboration between departments, and deteriorate the quality of care. Finally, strategies are offered to mitigate this bias through transparency, acknowledgment of interests, objective evaluation of proposals, and the use of neutral mediators.