We analyze the most relevant concepts about complexity of sciences, showing examples and consequences in medical sciences. In the first section we introduce some basic concepts about fractals and its relationship with the geometry of nature and chaos dynamics. We also introduce notions of critic systems, the role of energy in them and we propose a conceptual model for living systems. In this model we identify the non-linear dynamics as the dynamics of the living forms and fractals as the form resulting from that dynamics. In the second section we review medical and biological examples: infectious diseases, cardiovascular and behavior dynamics, from which useful concepts and medical hypotheses emerge. One of the most important concepts, is the proposition that homeostasis would have an irregular dynamic. Then, we could expect that healthy individuals should be characterized by irregular dynamics, but old or sick individuals would change, probably decreasing the complexity of its dynamics. Therefore, we ask ourselves if the use of therapies to obtain a constant internal medium can be a mistake.