[jamesmontalvo3]

I know almost nothing about genetics or virology, but perhaps GP is saying the close proximity of genetic similarity allows viruses to mutate and infect people it otherwise wouldn’t. For example, say Person A had a virus, and under no circumstances would Person A ever infect Person C due to Person C being just barely genetically different enough from A. Then we put Person B into the mix who is “halfway between A and C” (quotes because that’s probably not precisely how genetics works, but I don’t know). The virus infects B, then mutates and infects C.