It sounds like MG (Mycoplasma Gallisepticum). It's very contagious among chickens. If there was facial swelling AND a foul odor around the head area, I'd suspect Coryza. It's possible it could be both at the same time as well.
Treatment for MG is Tylan or Baytril. Treatment for Coryza is a sulfa drug such as Sulfadimethoxine or SMZ/TMP in combination with Tylan or Baytril.
I forgot to add; as Eggcessive mentioned, they will be carriers for life and will infect new incoming birds. You must maintain a closed flock. No new birds in, none to be sold nor given away. MG can be passed through eggs. No selling nor giving eggs away to be hatched. I recommend that you cull the sick birds and either incinerate them or bury them deep away from your coop/pens.
Wait at least 6 months before repopulating, that's after disinfecting your coop, feeders, waterers etc...