Question

What is lacking in a virus which makes it so dependent on living cells to multiply

Answer 1

The viruses lack crucial protein manufacturing mechanisms which are necessary for the process of replication.

The process of replication cannot take place if there is no production of the structural proteins to form the new set of cells. This is where the host cell ribosomes come in handy. The virus attack the host nucleic material and instructs the host cell to produce viral protein structure through the host tRNA,rRNA's,mRNA and the ribosomes. 

Therefore, the virus attacks the host cell replication mechanisms and uses them to make copies of itself.