Miasma outbreaks give every mob the same disease (#9232)

Content.Server/Chemistry/ReagentEffects/ChemCauseRandomDisease.cs

@@ -7,7 +7,7 @@ using JetBrains.Annotations;
 namespace Content.Server.Chemistry.ReagentEffects
 {
     /// <summary>
-    /// Default metabolism for medicine reagents.
+    /// Causes a random disease from a list, if the user is not already diseased.
     /// </summary>
     [UsedImplicitly]
     public sealed class ChemCauseRandomDisease : ReagentEffect

Content.Server/Chemistry/ReagentEffects/ChemMiasmaPoolSource.cs

@@ -0,0 +1,23 @@
+using Content.Shared.Chemistry.Reagent;
+using JetBrains.Annotations;
+using Content.Server.Atmos.Miasma;
+using Content.Server.Disease;
+
+namespace Content.Server.Chemistry.ReagentEffects
+{
+    /// <summary>
+    /// The miasma system rotates between 1 disease at a time.
+    /// This gives all entities the disease the miasme system is currently on.
+    /// For things ingested by one person, you probably want ChemCauseRandomDisease instead.
+    /// </summary>
+    [UsedImplicitly]
+    public sealed class ChemMiasmaPoolSource : ReagentEffect
+    {
+        public override void Effect(ReagentEffectArgs args)
+        {
+            string disease = EntitySystem.Get<MiasmaSystem>().RequestPoolDisease();
+
+            EntitySystem.Get<DiseaseSystem>().TryAddDisease(args.SolutionEntity, disease);
+        }
+    }
+}