Answer:because of their incomplete genome,they require host cell to complete their life cycle
Explanation:viruses are relatively unstable organisms and require the host cell machinery to complete it's cell cycle.
for example-hepatitis b virus infects a cell and then integrates its genetic code with the host cell thus whenever cell makes proteins,it also makes the viral proteins because of the integrated genetic material of virus in the host genome thus making it viable and easy to replicate by using host substrates.