This would mean that the phage is an epidemic disease and therefore it's strange that after two millennia there are still people that haven't been infected by the disease. The epidemic should be either eradicated or it should have reached everyone. The situation as described in simply impossible.
It mutates a lot, it's one mean mother.
The bigger question is why the Phage didn't infect everyone else, but the easy answer is because they didn't want it to take over the entire series.