Mycoplasma can kill, but many will survive. All poultry carry mycoplasma, so your other fowl will be carriers if you get new chickens. I would think about closing the flock to new birds, and treating sick birds. You could also just cull the sick ones, and see how they deal with the illness. There is no point getting new chickens if there is one guinea, duck, or turkey left. They will be carriers for life. You could check into the vaccine for mycoplasma (MG.)