Projectile_PlagueBullet.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using RimWorld;
using Verse;

namespace Plague
{
    class Projectile_PlagueBullet : Bullet
    {
        #region Properties
        //
        public ThingDef Def {
            get {
                return this.def as ThingDef_PlagueBullet;
            }
        }
        #endregion Properties

        #region Overrides
        protected override void Impact(Thing hitThing)
        {
            base.Impact(hitThing);

            /*
             * Null checking is very important in RimWorld.
             * 99% of errors reported are from NullReferenceExceptions (NREs).
             * Make sure your code checks if things actually exist, before they
             * try to use the code that belongs to said things.
             */
            if (Def != null && hitThing != null && hitThing is Pawn hitPawn) //Fancy way to declare a variable inside an if statement. - Thanks Erdelf.
            {
                var rand = Rand.Value; // This is a random percentage between 0% and 100%
                if (rand <= Def.AddHediffChance) // If the percentage falls under the chance, success!
                {
                    /*
                     * Messages.Message flashes a message on the top of the screen. 
                     * You may be familiar with this one when a colonist dies, because
                     * it makes a negative sound and mentioneds "So and so has died of _____".
                     * 
                     * Here, we're using the "Translate" function. More on that later in
                     * the localization section.
                     */
                    Messages.Message("TST_PlagueBullet_SuccessMessage".Translate(new object[] {
                        this.launcher.Label, hitPawn.Label
                    }), MessageSound.Standard);

                    //This checks to see if the character has a heal differential, or hediff on them already.
                    var plagueOnPawn = hitPawn?.health?.hediffSet?.GetFirstHediffOfDef(Def.HediffToAdd);
                    var randomSeverity = Rand.Range(0.15f, 0.30f);
                    if (plagueOnPawn != null)
                    {
                        //If they already have plague, add a random range to its severity.
                        //If severity reaches 1.0f, or 100%, plague kills the target.
                        plagueOnPawn.Severity += randomSeverity;
                    }
                    else
                    {
                        //These three lines create a new health differential or Hediff,
                        //put them on the character, and increase its severity by a random amount.
                        Hediff hediff = HediffMaker.MakeHediff(Def.HediffToAdd, hitPawn, null);
                        hediff.Severity = randomSeverity;
                        hitPawn.health.AddHediff(hediff, null, null);
                    }
                }
                else //failure!
                {
                    /*
                     * Motes handle all the smaller visual effects in RimWorld.
                     * Dust plumes, symbol bubbles, and text messages floating next to characters.
                     * This mote makes a small text message next to the character.
                     */
                    MoteMaker.ThrowText(hitThing.PositionHeld.ToVector3(), hitThing.MapHeld, "TST_PlagueBullet_FailureMote".Translate(Def.AddHediffChance), 12f);
                }
            }
        }
        #endregion Overrides
    }
}

RangedWeapon_PlagueGun.xml

<?xml version="1.0" encoding="utf-8"?>

<ThingDefs>

  <ThingDef Class="Plague.ThingDef_PlagueBullet" ParentName="BaseBullet">
    <defName>TST_Bullet_PlagueGun</defName>
    <label>plague bullet</label>
    <graphicData>
      <texPath>Things/Projectile/Bullet_Small</texPath>
      <graphicClass>Graphic_Single</graphicClass>
    </graphicData>
    <projectile>
      <flyOverhead>false</flyOverhead>
      <damageDef>Bullet</damageDef>
      <DamageAmountBase>9</DamageAmountBase>
      <Speed>55</Speed>
    </projectile>
	<AddHediffChance>0.05</AddHediffChance>
    <HediffToAdd>Plague</HediffToAdd>
    <thingClass>Plague.Projectile_PlagueBullet</thingClass>
  </ThingDef>

  <ThingDef ParentName="BaseHumanMakeableGun">
    <defName>TST_Gun_PlagueGun</defName>
    <label>plague gun</label>
    <description>A curious weapon notable for its horrible health effects.</description>
    <graphicData>
      <texPath>Things/Item/Equipment/WeaponRanged/Pistol</texPath>
      <graphicClass>Graphic_Single</graphicClass>
    </graphicData>
    <soundInteract>InteractPistol</soundInteract>
    <statBases>
      <WorkToMake>15000</WorkToMake>
      <Mass>1.2</Mass>
      <AccuracyTouch>0.91</AccuracyTouch>
      <AccuracyShort>0.71</AccuracyShort>
      <AccuracyMedium>0.50</AccuracyMedium>
      <AccuracyLong>0.32</AccuracyLong>
      <RangedWeapon_Cooldown>1.26</RangedWeapon_Cooldown>
    </statBases>
    <costList>
      <Steel>30</Steel>
      <Component>2</Component>
    </costList>
    <verbs>
      <li>
        <verbClass>Verb_Shoot</verbClass>
        <hasStandardCommand>true</hasStandardCommand>
        <projectileDef>TST_Bullet_PlagueGun</projectileDef>
        <warmupTime>0.3</warmupTime>
        <range>24</range>
        <soundCast>ShotPistol</soundCast>
        <soundCastTail>GunTail_Light</soundCastTail>
        <muzzleFlashScale>9</muzzleFlashScale>
      </li>
    </verbs>
  </ThingDef>

<!-- ====================== BASES ===================== -->


  <ThingDef Name="BaseGun" Abstract="True" ParentName="BaseWeapon">
		<techLevel>Industrial</techLevel>
		<smeltable>true</smeltable>
		<thingCategories>
      <li>WeaponsRanged</li>
    </thingCategories>
    <inspectorTabs>
      <li>ITab_Art</li>
    </inspectorTabs>
    <comps>
      <li>
        <compClass>CompQuality</compClass>
      </li>
      <li Class="CompProperties_Art">
        <nameMaker>ArtName_WeaponGun</nameMaker>
        <descriptionMaker>ArtDescription_WeaponGun</descriptionMaker>
        <minQualityForArtistic>Excellent</minQualityForArtistic>
      </li>
    </comps>
  </ThingDef>

  <ThingDef Name="BaseMakeableGun" ParentName="BaseGun" Abstract="True">
    <recipeMaker>
      <workSpeedStat>SmithingSpeed</workSpeedStat>
      <workSkill>Crafting</workSkill>
      <effectWorking>Smith</effectWorking>
      <soundWorking>Recipe_Smith</soundWorking>
      <recipeUsers>
        <li>TableMachining</li>
      </recipeUsers>
      <unfinishedThingDef>UnfinishedGun</unfinishedThingDef>
    </recipeMaker>
  </ThingDef>

  <ThingDef Name="BaseHumanGun" ParentName="BaseGun" Abstract="True">
    <weaponTags>
      <li>Gun</li>
    </weaponTags>
  </ThingDef>

  <ThingDef Name="BaseHumanMakeableGun" ParentName="BaseMakeableGun" Abstract="True">
    <weaponTags>
      <li>Gun</li>
    </weaponTags>
  </ThingDef>

  <ThingDef Name="BaseBullet" Abstract="True">
    <category>Projectile</category>
    <tickerType>Normal</tickerType>
    <altitudeLayer>Projectile</altitudeLayer>
    <thingClass>Bullet</thingClass>
    <label>bullet</label>
    <useHitPoints>False</useHitPoints>
    <neverMultiSelect>True</neverMultiSelect>
    <graphicData>
      <shaderType>Transparent</shaderType>
    </graphicData>
  </ThingDef>

</ThingDefs>

ThingDef_PlagueBullet.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using RimWorld;
using Verse;

namespace Plague
{
    public class ThingDef_PlagueBullet : ThingDef
    {
        public float AddHediffChance = 0.05f; //The default chance of adding a hediff.
        public HediffDef HediffToAdd = HediffDefOf.Plague;
    }
}