Violets parents didn't die from the virus. Both her parents were scientists who taught at Northview High school, whilst working on an experiment when Violet was 13 they were both killed. Her 18 year old sister looked after her until the virus hit, then she died.
Violet stayed in her house until people realised she was living alone, then she was taken by the privelliged but she escaped, then fled to the High School, and has been living there ever since. Until one day, the rest of the school kids found her.