I feel the same way as Doc Begely. I have no idea why the virus didn't infect both types of subjects as it should have.

Well, do you know why you feel that way? Because you're only half right. Viruses do indeed infect both plants and animals, but not at the same time. Plant viruses and animal viruses stay completely separate from one another. Plant viruses don't infect animals and animal viruses don't infect plants.