The idea of harnessing living organisms for treating human diseases is not new but, so far, the majority of the living vectors used in human therapy are viruses which have the disadvantage of the limited number of genes and networks that can contain. Bacteria allow the cloning of complex networks and the possibility of making a large plethora of compounds, naturally or through careful redesign. One of the main limitations for the use of bacteria to treat human diseases is their complexity, the existence of a cell wall that difficult the communication with the target cells, the lack of control over its growth and the immune response that will elicit on its target. Ideally one would like to have a very small bacterium (of a mitochondria size), with no cell wall, which could be grown in Vitro, be genetically manipulated, for which we will have enough data to allow a complete understanding of its behaviour and which could live as a human cell parasite. Such a microorganism could in principle be used as a living vector in which genes of interests, or networks producing organic molecules of medical relevance, could be introduced under in Vitro conditions and then inoculated on extracted human cells or in the organism, and then become a new organelle in the host. Then, it could produce and secrete into the host proteins which will be needed to correct a genetic disease, or drugs needed by the patient. To do that, we need to understand in excruciating detail the Biology of the target bacterium and how to interface with the host cell cycle (Systems biology aspect). Then we need to have engineering tools (network design, protein design, simulations) to modify the target bacterium to behave like an organelle once inside the cell (Synthetic biology aspect). M.pneumoniae could be such a bacterium. It is one of the smallest free-living bacterium known (680 genes), has no cell wall, can be cultivated in Vitro, can be genetically manipulated and can enter inside human cells.