Selective breeding/reproduction/engineering combined with evolutionary pressure in order to guide a genetic lineage in a particular direction. In the example of disease, say you wanted to make a human virus be able to infect mice.
You keep releasing a human rhinovirus into a cage with "humanized mice" (mice genetically altered to have specific receptors in common with humans). There is strong evolutionary pressure in this environment for the virus to evolve to infect these mice. A viral variant eventually catches on and begins infecting the humanized mice. Then you take that viral strain and start exposing normal mice. Now there is an evolutionary pressure for the virus to be able to infect those mice. This virus will have essentially "gained that function."
You keep releasing a human rhinovirus into a cage with "humanized mice" (mice genetically altered to have specific receptors in common with humans). There is strong evolutionary pressure in this environment for the virus to evolve to infect these mice. A viral variant eventually catches on and begins infecting the humanized mice. Then you take that viral strain and start exposing normal mice. Now there is an evolutionary pressure for the virus to be able to infect those mice. This virus will have essentially "gained that function."