Genetic engineering techniques like recombinant DNA technology involves the artificial introduction of genes that are originally not present in the cells of organisms. The introduction of these genes is used to create certain characteristics in the organisms that make them useful for humans.
Recombinant DNA techniques rely on the introduction of these foreign genes using what are called biological vectors. Viruses are an example of biological vectors that can be used. Viruses survive by altering the genes of other organisms by introducing genetic material into them that makes the organism produce more viruses.
Scientists use this property by introducing DNA into the virus that they want them to pass on into other organisms. A classic example of the use of recombinant DNA technology is the introduction of genes to produce insulin into E.Coli bacteria. This gives the bacteria the ability to produce insulin which is extracted for use by humans who have lost the ability to produce insulin on their own.