Although recent trends in politics and media make it appear that conspiracy theories are on the rise, in fact they have always been present, probably because they are sustained by natural dispositions of the human brain. This is also the case with medical conspiracy theories. This article reviews some of the most notorious health-related conspiracy theories. It then approaches the reasons why people believe these theories, using concepts from cognitive science. On the basis of that knowledge, the article makes normative proposals for public health officials and health workers as a whole, to deal with conspiracy theories, in order to preserve some of the fundamental principles of medical ethics. © 2020, Springer Nature B.V.