diff --git a/PKHeX.Core/Editing/Pokerus.cs b/PKHeX.Core/Editing/Pokerus.cs
index 0fff50ac3..f408a61b1 100644
--- a/PKHeX.Core/Editing/Pokerus.cs
+++ b/PKHeX.Core/Editing/Pokerus.cs
@@ -1,17 +1,92 @@
 namespace PKHeX.Core;
 
+/// <summary>
+/// Logic pertaining to Pokérus -- the virus that doubles EVs gained from battle.
+/// </summary>
 public static class Pokerus
 {
+    /// <summary>
+    /// Gets the max duration (in days) that a strain can be infectious.
+    /// </summary>
+    /// <param name="strain">Strain number</param>
+    /// <returns>Initial duration (in days). When the value decrements to zero, the Pokémon is no longer infectious.</returns>
     public static int GetMaxDuration(int strain) => (strain & 3) + 1;
 
-    public static bool IsObtainable(PKM pkm) => pkm is not PA8; // don't care about PK1
+    /// <summary>
+    /// Checks if any Pokérus values are possible to have on the input entity.
+    /// </summary>
+    /// <param name="pk">Entity to check</param>
+    /// <returns>True if Pokérus exists in the game format, or can be transmitted to the entity via another game.</returns>
+    public static bool IsObtainable(PKM pk) => pk is not PA8; // don't care about PK1, not stored in the data.
 
-    public static bool IsStrainValid(PKM pkm, int strain, int days) => IsObtainable(pkm) && IsStrainValid(strain, days);
+    /// <summary>
+    /// Checks if the Pokérus value for Strain is possible to have on the input entity.
+    /// </summary>
+    /// <param name="pk">Entity to check</param>
+    /// <param name="strain">Strain number</param>
+    /// <param name="days">Duration remaining</param>
+    /// <returns>True if valid</returns>
+    public static bool IsStrainValid(PKM pk, int strain, int days)
+    {
+        if (!IsObtainable(pk))
+            return IsSusceptible(strain, days);
+        return IsStrainValid(strain, days);
+    }
 
+    /// <inheritdoc cref="IsStrainValid(PKM,int,int)"/>
     public static bool IsStrainValid(int strain, int days) => strain switch
     {
         0 when days is not 0 => false,
         8 => false,
         _ => true,
     };
+
+    /// <summary>
+    /// Checks if the Pokérus value for Duration is possible to have on the input entity.
+    /// </summary>
+    /// <param name="strain">Strain number</param>
+    /// <param name="days">Duration remaining</param>
+    /// <param name="max">Maximum value permitted</param>
+    /// <returns>True if valid</returns>
+    public static bool IsDurationValid(int strain, int days, out int max)
+    {
+        max = GetMaxDuration(strain);
+        return (uint)days <= max;
+    }
+
+    /// <summary>
+    /// Checks if the Pokémon is immune to the Pokérus.
+    /// </summary>
+    /// <param name="strain">Strain number</param>
+    /// <param name="days">Duration remaining</param>
+    /// <returns>True if immune (cannot be infected).</returns>
+    /// <remarks>An immune Pokémon must have been infected and cured prior to being "immune".</remarks>
+    public static bool IsImmune(int strain, int days) => strain != 0 && days == 0;
+
+    /// <summary>
+    /// Checks if the Pokémon is currently infected with the Pokérus.
+    /// </summary>
+    /// <param name="strain">Strain number</param>
+    /// <param name="days">Duration remaining</param>
+    /// <returns>True if currently infected, and infectious to others.</returns>
+    public static bool IsInfectuous(int strain, int days) => strain != 0 && days != 0;
+
+    /// <summary>
+    /// Checks if the Pokémon can be infected with the Pokérus.
+    /// </summary>
+    /// <param name="strain">Strain number</param>
+    /// <param name="days">Duration remaining</param>
+    /// <returns>True if can be infected by another infectious individual.</returns>
+    public static bool IsSusceptible(int strain, int days) => strain == 0 && days == 0;
+
+    /// <summary>
+    /// Vaccinates the Pokémon so it will never be infectious in the format it exists in.
+    /// </summary>
+    /// <param name="pk">Entity to modify.</param>
+    /// <remarks>Overwrites all Pokérus values even if already legal.</remarks>
+    public static void Vaccinate(this PKM pk)
+    {
+        pk.PKRS_Strain = IsObtainable(pk) ? 1 : 0;
+        pk.PKRS_Days = 0;
+    }
 }
diff --git a/PKHeX.Core/Legality/Verifiers/MiscVerifier.cs b/PKHeX.Core/Legality/Verifiers/MiscVerifier.cs
index a1dbafbb8..8567eb801 100644
--- a/PKHeX.Core/Legality/Verifiers/MiscVerifier.cs
+++ b/PKHeX.Core/Legality/Verifiers/MiscVerifier.cs
@@ -130,9 +130,9 @@ private void VerifyMiscPokerus(LegalityAnalysis data)
             if (!strainValid)
                 data.AddLine(GetInvalid(string.Format(LPokerusStrainUnobtainable_0, strain)));
 
-            var expect = Pokerus.GetMaxDuration(strain);
-            if (days > expect)
-                data.AddLine(GetInvalid(string.Format(LPokerusDaysTooHigh_0, expect)));
+            bool daysValid = Pokerus.IsDurationValid(strain, days, out var max);
+            if (!daysValid)
+                data.AddLine(GetInvalid(string.Format(LPokerusDaysTooHigh_0, max)));
         }
 
         public void VerifyMiscG1(LegalityAnalysis data)
diff --git a/PKHeX.WinForms/Controls/PKM Editor/PKMEditor.cs b/PKHeX.WinForms/Controls/PKM Editor/PKMEditor.cs
index 85da376c5..cd5641bc8 100644
--- a/PKHeX.WinForms/Controls/PKM Editor/PKMEditor.cs	
+++ b/PKHeX.WinForms/Controls/PKM Editor/PKMEditor.cs	
@@ -1102,13 +1102,16 @@ private void UpdatePKRSstrain(object sender, EventArgs e)
 
         private void UpdatePKRSdays(object sender, EventArgs e)
         {
-            if (CB_PKRSDays.SelectedIndex != 0)
+            var days = CB_PKRSDays.SelectedIndex;
+            if (days != 0)
                 return;
 
             // If no days are selected
-            if (CB_PKRSStrain.SelectedIndex == 0)
+            var strain = CB_PKRSStrain.SelectedIndex;
+            if (Pokerus.IsSusceptible(strain, days))
                 CHK_Cured.Checked = CHK_Infected.Checked = false; // No Strain = Never Cured / Infected, triggers Strain update
-            else CHK_Cured.Checked = true; // Any Strain = Cured
+            else if (Pokerus.IsImmune(strain, days))
+                CHK_Cured.Checked = true; // Any Strain = Cured
         }
 
         private void UpdatePKRSCured(object sender, EventArgs e)
@@ -1141,7 +1144,7 @@ private void UpdatePKRSCured(object sender, EventArgs e)
             }
             // if not cured yet, days > 0
             if (!CHK_Cured.Checked && CHK_Infected.Checked && CB_PKRSDays.SelectedIndex == 0)
-                CB_PKRSDays.SelectedIndex++;
+                CB_PKRSDays.SelectedIndex = 1;
 
             SetMarkings();
         }