Viruses are inert outside of a host cell.
Viruses are cultured in the laboratory using cells of the appropriate host. Animal viruses are grown in tissue culture, on a monolayer or a suspension of cells. Bacterial viruses are grown in soft agar or in broth seeded with the host cells. Lytic viruses lyse the cell in order to leave. Thus they can be enumerated by making dilutions of the virus and assaying on a seeded agar surface for plaque formation.