Basically, once the virus has found a suitable host that is made up of the type of host cells that its virus type uses, it attach to the host. Then the viruses implant the key to its genetic makeup into the cell. When this RNA key of the virus is inserted into cells of a host, it allow the cells to be hijacked and at the appropriate time. If the virus stay inside your body in a long period, it can then turn the host cell into a factory for making mare viruses.