This chapter claims that we must know about health—not just illness or disease, and not even about the continuum health-disease. Now, in order to know about health, it is compulsory to know about life. After all, health is most probably the most important expression of life. To be sure, the sciences of complexity are precisely sciences of life. The inverse cannot, however, be assessed. The sciences of complexity make clear that grasping life entails knowing about life-as-we-know-it very much as life-as-it-could-be-possible. Such a twofold take brings to the fore that it is all about increasingly complex systems or behaviors. To say the least, health by any means be intervened, diagnosed, or determined, in any sense. Quite on the contrary, health, i.e., life, entails indetermination. Voilà, a counterintuitive statement. Vis-à-vis the health and the life sciences, health is adamantly not an anthropological or anthropocentric issue. Indeed, it largely overcomes the ongoing discussion about One Health, which is the currently standard and institutional discourse. In this take, the sciences of life are a propaedeutic for the health sciences. Hence, after, it becomes compulsory for the health sciences to steadily and strongly know about the sciences of complexity. This chapter does not depict what the sciences of complexity are. Rather, it focuses on the sense that health is a complex system. Surprisingly, health in general is a much more complex phenomenon than illness and disease—notwithstanding the wide and deep landscape of illness and the burden of disease. Arguments, proofs, and illustrations will be brought out that support the main argument.