Replace explicit z-level checks with defines

code/datums/diseases/advance/advance.dm

@@ -408,7 +408,7 @@
 			AD.Refresh()
 
 		for(var/mob/living/carbon/human/H in shuffle(GLOB.alive_mob_list))
-			if(!(H.z in GLOB.station_z_levels))
+			if(!is_station_level(H.z))
 				continue
 			if(!H.HasDisease(D))
 				H.ForceContractDisease(D)