The target will be "Poisoned" for the first three to five rounds.  Then diseased or not.  Not to bad. 

Yeah... the DND Design team sometimes make some absolutely dumb choices, like turning this spell into a useless waste of their ink and precious layout space. If they were thinking straight, they'd have just nerfed the over powered elements instead of that idiot choice of it not taking effect until three to five turns later. Seems that just because you work at Wizards in a decisionmakng capacity that doesn't make you good at making DND decisions.

Yet more bad design from 5e, nothing new

I think maybe this spell is better than people think. Yes a lot of  enemies are immune to poison, but if you find one that isn't, if you successfully hit with a melee spell attack then regardless of what happens for at least the next three roundsmost it's attacks will be at disadvantage due to poison. You can't legendary save against that melee spell attack, so it's a good way of causing poison in a target that has high save scores, with an added bonus of maybe doing something really cool if the enemy fails their con saves afterwards.

I don't understand why this needs to be a 7 days, 3 saving throws touch attack. Why not just do this:

Change the duration to 1 minute. Apply the poison as normal on a successful attack roll. Instead of rolling CON saves until it succeeds or fails 3 times, just make it continue for the duration so long as it keeps succeeding. Should it fail though, apply one of the diseases for the remainder of the duration. This means it has to succeed 10 CON saves to not get the disease. If that seems like too much, possibly make that part of it concentration, and have the concentration part removed once the disease takes hold, sort of like you're focusing to try and make them diseased but once it happens, you don't need to anymore. Also, possibly tweak Slimy Doom to be just incapacitated (no actions/reactions) instead of stun, so that the chances of a crit aren't massively increased for the duration.

Alternatively, just give the disease from the get-go and make them have to succeed 3 saving throws to remove it, no penalty for any amount of failed saves.

If they were making this with the intent of monsters using it against the players, just make it a monster exclusive ability that notes that it uses up a 5th level or higher spell slot.

Since it’s a natural disease, I don’t think it’s recommended. A 9th-Level player character is likely to have access to a paladin, and any serious creature at that level probably has the gold to get it cured. Unfortunate.

So I don't change much with how the spell takes effect, but I do let my players expand it. Mostly if they expend a sample vial of a disease as a material component, like crackle fever from the DMG, they can inflict that instead of what's listed. Raising the disease DCs to the caster's spell save DC.

Maybe if it was called 'choose your debuff' instead of contagion? Still pretty niche, but kind of an insurance policy against bosses with high save bonuses against something your party needs to be able to drop if the combat is still going and your resources are low.

Pick one ability score of your (boss?) target that you want to ensure disad on as the battle rages past the 3rd, 4th, 5th round. Players are already in some trouble if the battle is still going strong at this point. You need to know your party well enough to know what kind of Disad is most meaningful (Wisdom, Dex and CHA, which are the more common saves for high level spells, also have weaker disease effects- almost like they planned it that way :) )

Of course, since it's touch, it almost requires a familiar for remote delivery while the party is going through the minions - otherwise you're up close and personal, but by 9th level maybe you have some kind of misty step/cloak of the mountebank effect you can use (though maybe not on the same turn).

Under the right conditions you'll have a tough boss mob with at least a disad on the type of saves you need it to fail, if not an additional serious deficiency (stunned, confused, or vulnerability to all damage, etc), when resources are likely getting low for the party. But you need to know your opponent well enough to know if you can have those right conditions, or if this is partially or totally wasted. Would Juiblex use it on a Paladin? He's supposed to be smart enough not to. But he might hit the fighter with it, or the wizard, in early going, to basically land his Eject Slime and Acid Splash uncontested with them rolling DEX at a disadvantage.

It's definitely a lightning in a bottle scenario though and as others have said, likely better for monsters than players to use to any great effect.

i dont know why everyone is so upset, the absolute least this spell can do is poison a target for 3 turns.  is it the best use of a fifth level slot?  no.  does it have the potential to be one of the most broken spells if the enemy fails 3 saves?  absolutely.

Quote from Trewill91 >>

i dont know why everyone is so upset, the absolute least this spell can do is poison a target for 3 turns.  is it the best use of a fifth level slot?  no.  does it have the potential to be one of the most broken spells if the enemy fails 3 saves?  absolutely.

 People are upset because poisoned is really heavily resisted and this spell has a very interesting concept that as written gets reduced down to the poisoned condition. I'd personally rather see the spell apply the disease immediately and then just have slimy doom and flesh rot be less powerful. I personally apply the disease immediately unless I'm using this at lower levels.

Fun. Auto poison no save on hit. Thats a lot of tasty disadvantage.  or, making the big bad use their legendaries to break it. Good use 

Quote from Charmandenator >>

Quote from Trewill91 >>

i dont know why everyone is so upset, the absolute least this spell can do is poison a target for 3 turns.  is it the best use of a fifth level slot?  no.  does it have the potential to be one of the most broken spells if the enemy fails 3 saves?  absolutely.

 People are upset because poisoned is really heavily resisted and this spell has a very interesting concept that as written gets reduced down to the poisoned condition. I'd personally rather see the spell apply the disease immediately and then just have slimy doom and flesh rot be less powerful. I personally apply the disease immediately unless I'm using this at lower levels.

 Exactly! I'd houserule it to apply the disease immediately like before, except slimy doom gives vulnerability to piercing, slashing and bludgeoning damage, while flesh rot stops regeneration and deals necrotic and max hp damage every round (like mummy rot but stronger).

Is there any way that folks know of, to get this on the Alchemist spell list?

its a tasty debuff.. and while yeah plenty is immune to poison damage.  Immunity to poison damage isn't immunity to poison and poison condition.  As weird as that is.  Nothing i can find says they're the same. while i can find things like Yuan Ti race--that specifically calls out both damage and condition.  and nothing else in the game has immunity or esistence to damage apply to other tangential details.  

so.. pretty sure this affects evetything that isn't poison condition immune, or just 'poison immune'

While being immune to poison damage and being immune to the poisoned condition are different, few creatures have one but not the other.

Yep yep-though you shouldn't be trying to infct undead or automatons. though I wish you could devils/deamons-would be hilarious..  But it does help a lot with the Item based resistences.  Most poison related defensive items are Resistence.  Some others stated 'advantage against poisons' - such as belt of dwarven kind which gives adv vs poisons and resistence to damage..  I'm sure there are items that mention the condition as well, I just haven't actively come across them...  and have not really any true way to figure out how to search up items online anyay. 

You shouldn't use contagion against most of hte normal immune creatures anyway, but this difference does help vs humanoid races a little.  (Excluding yuanti ). Dwarfs are just ressistent to damage and adv vs poisons.  Which still really screws you up but still is a possibiltiy.   

Which.. ultimately is all I want out of it, the possiblity to use on a lot of things.  

This spell isn't correct as per the Player's Handbook.

It was changed in an errata.

Really want this on my Alchemist yep yep

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.

A lot of the people complaining about this spell being unusable ignore some of its applications. A Melee Spell Attack means that a creature (like a dragon) cannot use its Legendary Resistance to outright negate the spell upon casting, since Contagion doesn't require a Saving Throw until the end of the target's turn. Additionally, a successful save (such as through burning a Legendary Resistance if unable to make the save) doesn't rid the target of the spell: they are still poisoned, and still making saves until they make those 3 successes.

At a minimum, this spell is to ensure an enemy with Legendary Resistance remains poisoned (a nasty debuff in and of itself) and burns through those Resistances if it can't naturally make the save, making it vulnerable to your team's spells. At best, it not only forces the poisoned condition on the target regardless of saves for several turns, but inflicts a disease of your choice if it fails its saves. It's a long shot, but still a worthy addition to the arsenal of any caster willing to get close enough to inflict it.

Quote from WillJewwel >>

While being immune to poison damage and being immune to the poisoned condition are different, few creatures have one but not the other.

 The majority of creatures with an immunity to Poison are immune to the Poisoned condition. You can search for creatures immune to either on this site to confirm it.