Oath of Pestilence.

Clad in grime-soaked armor and wielding rusting weapons, the Oath of Pestilence paladin spreads corruption, disease, and filth. Bound by an oath which infests their bodies with all manner of plagues, these heralds of decay lumber forward with unholy toughness and grim resolve.

Level 1 Features:

Tenants of Pestilence.

The tenets of the Oath of Pestilence emphasize the role of disease as a purifying element in the cycle of life.
Strength in Resilience. Surviving hardship and plague make you stronger. Spreading these things causes strength to flourish.
All Things Must Pass. Death is the natural conclusion of life. There is nothing unnatural or amoral about the ending of life.
Might Makes Right. The laws of mortals mean nothing to poxes and plagues, they go where they wish and take what they want. So should you.

Level 3 Feature:

Oath Spells.

Each oath has a list of associated spells. You gain access to these spells at the levels specified in the oath description. Once you gain access to an oath spell, you always have it prepared. Oath spells don't count against the number of spells you can prepare each day.
If you gain an oath spell that doesn't appear on the paladin spell list, the spell is nonetheless a paladin spell for you.

Channel Divinity: Debilitating Fever.

Your touch inflicts disease. Make a melee spell attack against a creature within your reach that isn't a construct or undead. On a hit, the creature becomes diseased and is incapacitated for 1 minute. At the end of each of the diseased target's turns, the target can make a Constitution saving throw. On a success, the target is no longer diseased, and the effect ends.
Since this ability induces a natural disease in its target, any effect that removes a disease or otherwise ameliorates a disease's effects apply to it.

Channel Divinity: Entropic Infection.

You can use your Channel Divinity to weaken a creature against entropic energies. As an action, you can force a creature of your choice that you can see within 30 feet of you to make a Constitution saving throw. On a failed save, for the next minute the creature takes an additional 2d6 damage the first time it takes necrotic damage each turn. During this time, the creature loses resistance to necrotic damage if it has it.

Level 7 Feature:

Aura of Rampant Sickness.

Starting at 7th level, you emit an aura of contagion and virulence to a range of 10 feet. When a creature within your aura makes an ability check, attack roll, or saving throw you can give that roll disadvantage as a reaction. Once you do, you can't give another roll disadvantage using this feature until the start of your next turn.
At 18th level, the range of this aura increases to 30 feet.

Level 15 Feature:

Disgusting Resilience.

At 15th level, when you take damage which would reduce you to 0 hit points, you can choose to expend any number of hit dice. Roll those dice and subtract the result from the damage taken. If this reduces the damage to 0, you don't take any damage.
In addition, if you are killed your corpse violently explodes in a shower of pus and gore, dealing 8d6 necrotic damage to creatures within 20 feet of you that fail a Constitution saving throw.

Level 20 Feature:

Plaguebringer.

At 20th level, you become an avatar of plague, which gives you the following benefits:
-You have resistance to necrotic damage and are immune to poison damage and the poisoned condition.
-Your hit point maximum can't be reduced.
Creatures of your choice who start their turn within 5 feet of you, or who move within 5 feet of you on their turn, take necrotic damage equal to your Charisma modifier. Once a creature takes damage from this ability, it can't take damage from it again until the start of its next turn.