"There is enough in the world for everyone's need, but not enough for everyone's greed".
An example of genetic conflict has been studied in Oryctolagus cuniculus (rabbits) that they are infected with myxomatosis virus, whose virulence level will be important to transfer from one host to another.This virus causes lumps around the head and genitals. It then may progress to acute conjunctivitis and possibly blindness; however, this also may be the first indication of the disease. The rabbits become listless, lose appetite, and develop a fever.
Secondary bacterial infections occur in most cases which cause pneumonia and purulent inflammation of the lungs. In cases where the rabbit has little or no resistance, death may take place rapidly, often in as little as 48 hours.
When the genetic variability is high and each host is infested by different strains viruses, natural selection will favor the virus that they are able to reproduce more quickly. Rapid proliferation of the virus involves a greater utilization of the resources of the host, causing a negative effect faster and direct about the rabbit, so the competition between viruses within a host selects a greater virulence.
However, if the host dies prematurely and viruses don't have the time required for transmission to other hosts, most of the virus die and the "beneficial" fitness will not proliferate.Thus, natural selection will favor lower virulence levels that allow efficient transmission the virus to other hosts. So, selection individual level favors those who get maximize their reproductive rate, but there would a selection at the group level that favor a decrease in the replication rate because this character makes the virus have a more efficient dispersion to other hosts.
In conclusion, the virus most efficient are those having average levels of virulence.
Soler, Manuel. "¿A qué nivel actúa la selección natural?" Departamento de biología animal y ecología. Facultad de Ciencias. Universidad de Granada.
Carlos María Rodríguez López