They are simple and robust little demons. If we want to make lots of a mammalian virus, we take a tiny pinch of pre-isolated virus from a vial in the freezer or introduce its genes separately via plasmids into a layer of mammalian cells in culture. The pre- made virus will infect the cells, hijack the cells' machinery to replicate itself, and the cell culture medium will begin to load up with new virus particles. For the case when we have transfected the virus genes by way of electroporation or lipid vessicle method, the viral genes express inside the mammalian cell and the new virus will self assemble. When they emerge from the cell they begin infecting other cells and replicate. The cell culture medium then loads up with new virus particles. After all the feeder mammalian cells are dead, we take the loaded medium and freeze aliquots, or if we want a very pure isolate of virus, we use gradient centrifugation, remove the layer containing the pure virus with a pipet, then dialyze the virus particles into a different buffer of choice by way of a semi permeable membrane.