How do i find the mothership?

lol my 9yr old girl has come home with head lice, the first time was in october i kept her home bought nix etc, after that she was clean , now i find them on her again ,i as a parent know it happens but it pisses me off that i have to keep doing this it is hard to keep her still and she has a headful of hair ,not to mention the whole treatment is kind of steep ,besides telling the school nurse how can i find out who is infesting her, not being a b**** but ,i want to know who it is so she can steer clear of them ,ps i dont have em and i washed , linens ,brushes etc