im pretty sure there is an error correction mechanism that removes genetic sequences which seem out of place or mutations. no idea how likely it is to always work, but infected cells are cleaned up through autophagy, programmed apoptosis (this is like murder and suicide), so viral sequences will most likely be removed at some point. chronic viral infections like aids and herpes are forever though. edit: i do not have a degree or anything in this area, so if you want to know more im sure youtube has simple videos explaining these things