All viruses attack their hosts and introduce their genetic material into the host cell as part of their replication cycle. This genetic material contains basic 'instructions' of how to produce more copies of these viruses, hijacking the body's normal production machinery to serve the needs of the virus (see figure 1). The host cell will carry out these instructions and produce additional copies of the virus, leading to more and more cells becoming infected. Some types of viruses actually physically insert their genes into the host's genome. This incorporates the genes of that virus among the genes of the host cell for the life span of that cell.

Viruses like this could be used as vehicles to carry 'good' genes into a human cell. First, a scientist would remove the genes in the virus that cause disease. Then they would replace those genes with genes encoding the desired effect (for instance, insulin production in the case of diabetics). This procedure must be done in such a way that the genes which allow the virus to insert its genome into its host's genome are left intact.

Figure 1. How do viruses work? Recommended: Learn more about viruses by viewing a very informative video called "Understanding viruses" (17 parts, total time 43:35, video will open a new window).

Many gene therapy clinical trials rely on retroviruses or adenoviruses to deliver the desired gene. Other viruses used as vectors include adeno-associated viruses, lentiviruses, pox viruses, alphaviruses, and herpes viruses. These viruses differ in how well they transfer genes to the cells they recognize and are able to infect, and whether they alter the cell’s DNA permanently or temporarily (see figure 2).

Figure 2. A comparison of different viral vectors in use for gene therapy: overview of their advantages and disadvantages. * Adeno-associated viruses are able to integrate with low frequency into chromosome 19. Lentiviruses also infect non-dividing cells.