The rabbits would be selected for resistance to the virus. Those 0.2% which survive infection must have something different about them genetically which allowed them to survive/be selected by nature. Only these rabbits survive to would contribute to the next generation, making the next generation genetically much more resistant to the virus. This could be due to membrane protein structure if myxoma tricks its way into rabbit cells by binding to cell surface proteins.
Work Step by Step
You could also describe how the virus would evolve to try to overcome any resistance put up by the rabbits, with altered gene sequences (leading to altered protein structure/function) being selected if it helped in overcoming resistance by the rabbits.