Does the Contagion spell infect the target immediately on a melee attack hit?

Question

Let's say I'm a cleric with the 5th level spell Contagion and I am successful on the melee attack. From the wording of the spell it seems that the effect kicks in immediately, but Jeremy Crawford said that it "activate[s] after three failed saving throws."

Josh‏ @SirJosh94 · 27 May 2015
@JeremyECrawford Do contagion effects kick in immediately or do they kick in when the target fails 3 saving throws?

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Jeremy Crawford‏ @JeremyECrawford · 29 May 2015
The effects of the contagion spell's disease are meant to activate after three failed saving throws.

I agree with him on that part, that the disease chosen would continue or not based on the success or failure of three saving throws.

But I think that the wording is pretty clear:

Your touch inflicts disease. Make a melee spell attack against a creature within your reach. On a hit, you afflict the creature with a disease of your choice from any of the ones described below.

So which is true?

Does Contagion inflict the disease immediately, and the saves are just for determining whether the disease will become permanent for 7 days?

Answer 1

As per 2018 PHB Errata, Contagion now applies poison before it applies disease; RAW, the disease is only applied after 3 failed Constitution Saves.

New Contagion:

Your touch inflicts disease. Make a melee spell attack against a creature within your reach. On a hit, the target is poisoned.

At the end of each of the poisoned target's turns, the target must make a Constitution saving throw. If the target succeeds on three of these saves, it is no longer poisoned, and the spell ends. If the target fails three of these saves, the target is no longer poisoned, but choose one of the diseases below. The target is subjected to the chosen disease for the spell's duration.

Answer 2

This answer was accurate as of when it was originally written, but the description of the spell was changed in the 2018 PHB errata.

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Rules as Intended: Contagion takes effect after the three failed saves

This tweet by Jeremy Crawford and this Rules Answer article from June 2016 both say Contagion takes effect after the third failed save. Apparently, enough groups were playing correctly given the spell description that it simply didn't make it into the errata.

Crawford does say that if he could change it now, he would, as more groups seem to not be playing according to RAI.

RAW: The disease takes place immediately

The following line says on a hit.

Your touch inflicts disease. Make a melee spell attack against a creature within your reach. On a hit, you afflict the creature with a disease of your choice from any of the ones described below.

The following line, from the next paragraph, also imply the disease has already been afflicted:

After succeeding on three of these saving throws, the creature recovers from the disease, and the spell ends.

If there was no disease, what would there be to recover from?

What may have happened

As noted by @SevenSidedDie in a comment on this answer:

Maybe the thinking is that having disease ≠ suffering overt effects of a disease, so the incubation time between contracting the disease (the hit) and showing symptoms is represented by three failed saves, and contracting the disease but fighting it off before showing symptoms is the three succeeded saves? It's a reading that seems non-obvious to me too, except in hindsight knowing the intended reading, and that's the only rationalisation I can think of for the RAW after learning of the intended meaning.

So they may have intended an incubation period for this disease. However, the spell does say "on a hit, you afflict [...] a disease of your choice" and listed choices do not mention an incubation time. As such, this is a justification you can present when deviating from RAW.

But the concept of incubation does explain why they would have worded it this way, and in my opinion, it's likely what they were thinking when they wrote the spell.

Answer 3

My view is that if it was meant to have/be incubating for three rounds they would have not left it open to this much interpretation. It would have been worded in such a manner to sound something as this * upon a successful melee strike you infect the target and the disease incubates until either the target succeeds on three saving throws at which time they are cured of the malady, or fail the saving throws at which time they succumb to the malady and begin to suffer the effects. *. They worded in a way to make me think that it starts on the successful melee strike and continues until the target makes the save or they die while suffering its effects or ruin the timer out. This is just my two cents as a current player and former DM / GM, I will also say this is how my current DM also interprets this spell.