My chicks injured a younger one!

[Chickenfirster]

I have several chicks right now in my brooder, and it is my first time ever raising chickens. I have a little polish chick in with the other chicks (rhode island red, leghorn,) and yesterday everything was fine. Today though, I went out and checked on them and they had been pecking at her head and now the tuft of feathers is almost gone and she has blood all over her head How should I medicate this and proceed???