Viruses are associated with the most devastating illnesses and epidemics threatening all of humanity and thus have a horrifying image, but is it fair to simply brand them all with this face of evil? Some viruses do not provoke illnesses, so is there in fact another side to the beast? This entire film tells us how these micro-organisms, for so long simply unknown, have gradually revealed their incredible capacities, and this time for the greater good of humanity. Today we know that the world of viruses is closely linked to our evolution. Almost half of our gene pool is made up of sequences of viral origin. Some viruses can indeed be precious tools for treating genetic diseases or even cancers. What is yet more surprising is that the presence of a virus we all carry in our gene pool is needed for forming the placenta. The virus world offers an incredibly wealthy source of lessons and it is far from having revealed all its mysteries.