…or 776 reasons it got buffed.

Back in June, we dove into Contagion spell odds. The analysis came on the heels of comments by Lead Rules Designer Jeremy Crawford indicating that Contagion’s disease was not intended (RAI) to take effect until the target failed a best-of-5 save mechanic, seemingly contrary to the plain language of the rule (RAW). The Player’s Handbook errata released in November 2018 patched this gap by inflicting the poisoned condition.

The Problem

Problem with RAW. As a quick refresher on its original form, Contagion inflicted disease upon a successful melee spell attack. This dropped you into a best-of-5 Constitution save mechanic which either alleviates the disease on success, or causes it to be permanent on failure. Disease-on-hit provided a false choice among the 6 Ability Score-targeting diseases Contagion can inflict, since Slimy Doom gives disadvantage on Constitution saving throws, making Contagion more likely to stick around.

Problem with RAI. Recognizing this design flaw, Crawford tweeted that the intention was for the disease to not take effect until the resolution of the save mechanic. While this eliminated the false choice, it also eliminated any viability of Contagion as a spell, since it was now locked behind the most onerous mechanic in the game before having any effect.

The Errata

The 2018 errata patched this mechanic by applying the poisoned condition upon a successful attack roll, and pushing the disease back to the end of the save mechanic, as intended.

This is a really elegant solution. While the poisoned condition inflicts disadvantage on attack rolls and ability checks, saving throws fall into neither of these classifications, removing the hazard that Contagion makes itself more likely to trigger.

Unintended Consequences

Even the most elegant solutions must live with the Law of Unintended Consequences. While the poisoned condition is a nice puzzle piece to plug the Contagion gap, poison and disease are different, and therefore interact differently with the creatures subjected to them.

Disease. Disease does not have robust treatment in the rules of 5th Edition Dungeons & Dragons. There is no diseased condition. In a recent Happy Fun Hour stream, D&D Franchise Creative Director Mike Mearls noted that the Paladin’s disease immunity is considered a “ribbon” ability, since disease is not perceived as a big threat in the game. Disease existing mostly outside the rules is part of what made Contagion such an attractive option to spellcasters.

Poison. Poison is one of the most resisted damage types and condition immunities in the game. There are currently 271 published creatures with immunity to the poisoned condition. Spellcasters relying on poison are going to have a bad time against constructs, elementals, fiends, green dragons, undead, and yuan-ti.

Ultimately, whether Contagion got stronger or weaker is going to depend on how you were running it before. If you were running Contagion RAW, then it’s been nerfed because a lot more creatures are immune to poison than disease. Poison is still a pretty strong effect, but many of the disease choices are even stronger, and affect saving throws, as the poisoned condition does not. If you were running Contagion RAI, the errata is a clear upgrade because you’re inflicting the poisoned condition for 3-5 rounds, where previously you got nothing.

Errata Drawbacks

Based on the rewritten spell, creatures with poison condition immunity will never suffer the disease effects of Contagion. Only a poisoned target is required to make the saving throws to determine whether it becomes affected by the chosen disease.

Note: In a strange result, the poison-immune target is still afflicted by a disease that has no effect, since the save mechanic never resolves. This raises the question of whether the poison-immune targets are perpetually infected with an innocuous disease. Perhaps these creatures are unknowingly spreading a plague in your campaign world like Typhoid Mary.

Errata Benefits

Where Contagion excels is burning Legendary Resistances of the target. Whether playing the original RAW or the new errata, the target is going to be affected by disease/poison for 3-to-5 rounds, even while burning through Legendary Resistances. Since the target makes the save at the end of their turn, they’re going to suffer the effects of poison for 3 rounds minimum.

The Odds

Having explored the mechanical benefits, let’s take a dive back into the odds and figure out how the new fiddly bits of Contagion work. We’ve updated the Contagion calculator to show you how long the poisoned condition is going to last. Keep in mind that these odds assume you’ve already made a successful melee spell attack against the target.

Contagion Calculator

Here you see the odds for an 11th-level spellcaster with a maxed ability score that’s just learned Contagion. Feel free to tinker with the calculator in the link to view the odds of poison duration for different builds. The calculator doesn’t include functionality for disadvantage, since Slimy Doom was stripped away in the errata and it’s tough to stack with Bestow Curse. If you want to read more about how disadvantage affects Contagion odds, check out the original article.