Richard, duplicating yourself upon others is never so easy. We, whether viruses of DNA or the mind, face the hassle that cells and minds do not like being duped and abused. Imagine what it is like being a DNA virus. Cells might have replication machinery waiting there to copy you but cells also block access by surrounding themselves with membranes. And entry is only the start of your problems.
Cells make existence tough against foreigners.
Worse, the bodies they make long ago evolved to spot and destroy any cells viruses had taken over. Take entry, cells have a transport system across their membranes in the form of receptors. Packed in our survival gear are kidnapped proteins which link with them and so like forged passports let us in. We have more problems once inside a cell. We need to stop them being alerted.