This is an older story. Sam and Dean are after a monster in the woods when they discover they are being hunted. It's actually a group monsters. They are hunting people for food or turning them into clan members. Something like a fungus is the process used. Sam gets hit and is slowly turning. It hurts when Dean tries to remove the growths but fire hurts it. At one point the boys find a temporary refuge while the monsters are trying to hit them with the fungus type stuff