I read a book in English class at school around 1997/1998, about a girl who is the only survivor in a post nuclear war. The girl lives on a small farm and has somehow managed to avoid radiation, she has a .22 rifle for hunting/defence. There is a man in a Biohazard suit that turns up at her house and she believes he is a scientist, he turns out to be a murderer that stole the suit and made his way out of the war zone to the farm. The girl realises this as she finds bullet holes in his suit once he has taken it off and hung it up in the house. It then becomes a fight for survival for the girl... We didn't finish the story at school for some reason, I can't remember any of the names to help either.
This sounds like Z for Zachariah by Robert C. O'Brien. Some of the details are slightly different (the man in the suit is actually a scientist) but the broad outline matches your description.