Recombinant virus research. Samples of genetically engineered (recombinant) viruses being frozen in liquid nitrogen for storage. Viruses are used as vectors for introducing new genes into cells. Once a virus infects a cell it inserts its own genes (DNA) into the cell's genetic material, causing it to produce more viruses. The virus can be genetically-engineered so that when it infects a cell, it inserts DNA that causes the cell to produce a specific chemical. Bacteria may be infected to produce novel chemicals, whilst the virus may be used on humans (gene therapy) to make their cells produce an essential chemical & so cure a disease.