They are genetically fucked up in many ways we all know the typical shit no empathy inbreeding big nose no soul etc etc... that is the parasite aspect. Their religion is the virus the thing that makes a lot of their fucked up issues not only possible but helps it spread. They think they are the choosen people for example.. it would take an entire book to fully flesh this theory out but I'm sure I will get some interesting takes on here about it.