Both the viral and host genes determine virulence. The scientist can compare the virulence of different types of the same virus with one another; but it is impossible to compare the virulence of two different viruses. To study the roles that different genes play, scientists make mutations in the genes of interest; and then they are tested in animals. As expected, the virus shouldn’t affect the growth of cells in cell culture; but as expected when the virus (now carrying the mutant gene) is put in the mouse, it’s attenuated. Virulent viral genes affect viral replication, invasiveness, and the ability of the virus to infect different tissues. Virulent viral genes also are able to modify host defense mechanisms; which allow the virus to spread throughout the host.
The viral disease, LCMV, is mediated by CD8+ cytotoxic T lymphocytes. When LCMV is injected into normal mice, the mice are dead within eight days. If the scientist injects mice without an immune system, the mice live. This indicates that the immune response is what’s killing the mice, in this particular disease. If the mice without an immune system injected with LCMV were injected with T cells as well, the mice would die. Very young and very old people seem to be more susceptible to diseases caused by viral infections than the rest of the population. Infections of the host by a virus undergo an incubation period. During this incubation period, some amount of time goes by before the symptoms usually caused by the virus presents themselves. During the incubation period the viral genomes are replicating; and to some extent the host is responding. It has not yet been determined whether the virus will be transmitted or not. It has been found that incubation periods vary for different viral infections.