Complexity appears in the twentieth century as a way to understand many phenomena that are perceived as chaotic and complex from classical thought, which still persist in our way of explaining the world. Its purpose is to study the complex and adaptive systems that are sensitive to initial conditions. Some of the characteristics of complex thought are systemic perspective, autopoiesis, self-organization, emergent properties, unpredictability of the systems, analogic thought, and the complementarity of the phenomena, among others. Living systems respond to a complex logic, and in that sense, our vision of human populations and patients, and how we try to solve problems and human diseases, should be open to the possibilities that arise from this form of understand the world.