N 9 Viruses use the host's machinery to make copies of themselves. However, some human viruses require a type of replication that humans do not normally do. For example, humans normally do not have the ability to convert RNA into DNA. How can these types of viruses infect humans, when human cells cannot perform a particular role that the virus requires?

This is because the Viron has in its genome a specialized code for synthesising any missed enzyme for replication that is lacking in the host cell.

An example is the replication of the human DNA cells by the Immunodeficiency virus (HIV). Human cells only have enzymes for copying DNA templates, and lacks the enzyme to convert the HIV RNA genome to human DNA. However these viruses have in its genome; code for synthesising its own RNA polymerase enzymes that copies or transcribed the human DNA to HIV RNA.

This ability of the viral cell to code for the host's enzyme has a therapeutic effect. Drugs can be targeted at the viral polymerase enzymes to reduce the replication and therefore toxicity in the host cells.