Genes are composed of Deoxyribonucleic acid – DNA. DNA is composed of genetic information, by which they produce the specific proteins which are the building blocks of the body. 
  

Gene damage or missing gene may lead to inadequate or wrong protein production leading to disease condition.
  

In gene therapy, a normal gene is inserted into the genome to replace an abnormal disease causing gene.
  

A carrier molecule “vector” can be used to deliver the therapeutic gene into the patients target cells.
  

Viruses are microorganisms which infect humans and multiply in the genome of the human body.
  

Scientists have tried to take advantage of this multiplication capacity of the viruses to transfer the required genes into the human body for treatment purposes.
  

The virus is manipulated by the scientists and the pathogenic disease causing genes in the virus are removed and the therapeutic genes are inserted.
  

The virus vector then unloads its genetic material containing the therapeutic human gene into the target cell (human host).
  

The treated human produce the normal proteins due to the transfer of the therapeutic genes.