Why can't Mattie separate herself from Nell?

The fever of the title has been ravaging Philadelphia, bringing death and suffering to so many people. One of the victims of this terrible disease is Nell's mother. Nell's only a little girl and so losing her mother at such a young age has left her a homeless orphan. Mattie sees her crying in the street one day, desperate for someone to help her. Mattie takes pity on Nell and instead of putting her in an orphanage takes her back to her abandoned house.
Mattie cares deeply for Nell, looking after her when she too comes down with the same yellow fever that killed her mother. Over time, Mattie develops a very close bond of affection with Nell, treating her as if she were her own child. It's no wonder that she can't separate herself from Nell, and it's even less of a surprise when she makes their special relationship formal by adopting Nell and making her one of the family.