Dang, the comment section is even more toxic than this spell!

its a spell that has a chance to turn every hit into a stun. so think power word stun with a chance of effect. you know the 8th level power word stun.

I don't understand where all the hate for this spell comes from, it is really quite potent if you know how to use it.

First of all, the poisoned condition is really useful, giving disadvantage on attack rolls effectively gives a +3 AC to your entire party. The disadvantage on ability checks lowers a creatures Passive Perception by 5, allowing Rogues to hide more easily.

There are only 2 spells in the game that cause the poisoned condition, the other being Ray of Sickness. While that spell is a 1st level spell, it only inflicts poisoned after a sucessful hit and a sucessful CON saving throw, and even then it only subjects the target to the condition until the end of your next turn, wheareas this spell poisons the target for at least 3 turns.

The argument that there are a lot of monsters with the poison immunity is valid but misleading.

• All Fiends are immune to poison.
• All Constructs are immune to poison.
• All Undead are immune to poison.
• All Elementals are immune to poison.

If we look past these monsters, we find that there aren't quite so many monsters that are immune to poison.

• Dragons: Only the green dragon is immune to poison.
• Beasts: There are only 2 beasts immune to poison, the strongest having a CR of 1/2.
• Abberations: There are 6 abberations immune to poison, 3 of which have a CR of 20+.
• Celestials: There are 5 celestials immune to poison, 4 of them legendary, 2 of them having a CR of 20+.
• Fey: There are 6 fey immune to poison, 5 of them came out this year (4 from Theros, the 4 nymphs, the other in Tasha's), the last one is a legendary monster from Wildemount.
• Giants: There are 4 giants immune to poison.
• Monstrosities: There are 21 monstrosities immune to poison, 7 of which are the Yuan-Ti, 5 others clearly resemble snakes.
• Oozes: There is 1 ooze immune to poison.
• Plants: There are 3 plants immune to poison.
• Humanoids: There are 21 humanoids immune to poison, the majority of which are characters from adventures. The remainder is made up mostly of Grung and Quaggoths.

Finally, most of these monsters that are immune to poison, are from the Out of the Abyss adventure.

All in all, only a fraction of the monsters to which you would uses this skill is actually immune to poison, and most of those are so for obvious reasons.

Besides all this, as mentioned in earlier comments, this spell has also some nice roleplaying applications, poison someone while he is walking down the street. Before he realized what happened (30 seconds at the most), he'll be afflicted by the disease, and your party can ambush him with ease.

Great villain spell. If you are playing an evil character and your party does not have a paladin or some other class that will heal you with a spell neutralizing the disease, this could be deadly. You could literally plant this spell (as a villain) in another creature(think of bag of rats and let them go in the streets) and let them nibble on the population...this would be awesome if you're planning on playing anything close to a plague doctor. I haven't actually played with this spell yet, but I understand the potential and the gripe. Poison is heavily resisted at higher levels so it's not easy to pull of. If disease had its own damage type it might make it interesting. The wait time for this spell to pop off is also (in my opinion) meant to act like a dormant virus that takes time to take over the body. Makes sense, but mechanically it's a waste of time. If there were more options to make poison, and disease more potent to overcome these obstacles that would make it easier. Would a creature you summon like find familiar, be able to spread it around? Disease and poison need to be buffed but I like the flavor of the spell, but not the execution of it. Make it 1 saving throw either a set DC of 18+ or a CON saving throw against the casters spell save DC. I've thought about homebrew spells that cause disease like having mosquitos, rats, and other animals that carry diseases. These are some of my thoughts. 

There is something to be said about the fact that it still inflicts the poisoned condition which is a rather useful debuff in it's own right, and then can transition into something even more powerful later on. 

Okay but has anyone thought about twin casting the spell with the Divine Soul sorcerer either on two enemies or one really unfortunate person?

Being someone with an actual Seizure disability, I think Seizure should also impose the incapacitated Status or 1 point of Exhaustion.

why doesn't the wizard have this spell? 

My problem with this spell is that it's called "contagion" but it has nothing to do with being contagious. It's a completely missed opportunity for a really unique and flavorful mechanic. Imagine you cast contagion on a creature and each turn it coughs or vomits in a random direction spraying a 15' cone or a 30' line (respectively) and other creatures hit have to save or become infected. Alternatively or if other creatures start/end their turns within 5-10' of an infected creature, they have to save or become infected. You could spread a disease around the battle field. You could use control spells or pushing and shoving or whatever to force contagious baddies close to others and cause a faster spread.

The effects obviously couldn't be so powerful, but that's the direction I would have liked them to take with this spell. Otherwise, just call it "sicken" or something.

Reminder to all, that this spell on a hit makes the target poisoned for a minimum of 3 rounds, possibly 4 or 5. That not bad, especially since the spell doesn't require concentration. The only other spell that makes a creature poisoned is ray of sickness, and that only lasts for 1 round max.
So casting this spell and not getting the game-breaking contagions in effect might feel like losing out on a golden trophy, but the race to find out is still going to net your some good combat control anyway.

Twinned Spells can't target one creature twice. Great strategy to make the most of it though, by twinning it!

A contagious seizure sounds so scary as somebody whos had a seizure

Pretty sure this spell is supposed to be used against the player, which makes it much more devastating. Stop complaining!

I do wish 5e handled diseases a little more clearly.

There are numerous creatures that are NOT immune to disease that ARE immune to poison and the poisoned condition.  Because of those poor mechanics these poison immune creatures will be immune to this spells effects.

Due to the wording not being poisoned means the constitution saves never have to be made.  Thus the stronger effect never kicks in because poison immune creatures that are not immune to diseases are immune to this spell.

It's much better than the original wording, but if a player at my table picks this up only creatures that are disease immune (paladins and the like) are going to get a pass on this.

No the big, bad poisonous snake is not going to be immune to your DISEASE inflicting spell.

Agreed, this spell needs an errata clarifying that poison resistance/immunity to the poisoned condition does not protect a creature from this spell unless the creature also has resistance or immunity to disease.

As a DM, it's primarily used by bad guys, and is almost used outside of combat. However, many people are missing a very important part of the spell, the poisoned condition.  On a hit, the target is poisoned for a minimum of 3 turns, possibly 5, and can then contract the disease.  Poisoned giving disadvantage on attacks and ability checks is quite nice for 3-5 turns.  Though as I said earlier, I use it primarily outside of battle. A stranger bumps into you "accidentally" in a crowded market place (as well as others) attempting to spread a plague throughout the town. A minute goes by, and now the whole square is in a panic.

• All Fiends are immune to poison.
• All Constructs are immune to poison.
• All Undead are immune to poison.
• All Elementals are immune to poison.

I just wanna be nitpicky, but most of those do have exceptions, so it's not "all", but "most".

A few examples; succubi/incubi are only resistant to poison dmg and can be poisoned, as can vampires who aren't even resistant to poison dmg.

Slimy doom with Circle of Spores Druid is OP, enemy with disease approaches you? Ohh make a con save with disadvantage and get stunned if you fail.

The fact it's a default spell for three subclasses does suggest otherwise, but to my mind where this spell now in large part works well is as a 'DM spell'; not nearly to the same degree, but similar to how other spells like sequester and instant summons have such odd-seeming usage from a player perspective. To my mind, an early hit in a lengthy boss encounter where a PC just seems innocuously poisoned but that starts a doom-timer would be a brilliantly dramatic aspect of risk. Even better, I imagine, if the players aren't familiar with the spell, and the DM starts mysteriously asking for saving throws that don't seem to do anything (...at first!)

If this still seems such a missed opportunity, bestow curse offers a comparatively flexible touch-debuff (admittedly less-powerful but still fantastic) that takes effect instantly, that also doesn't use concentration when upcast to the same level as this spell.

Didn't see it mentioned in the comments here, but as a touch spell you can cast this through find familiar, which makes it much easier to set this up ahead of a fight.

If an adventurer comes running in and happy slaps you with a slimy doom hand then obviously, yeah, you're going to be on high alert. But if a smaller critter creeps in and bites you you're going to assume the small critter is what poisoned/diseased you.

The main drawback in this case is going to be how much access the enemy has to removing diseases; for your average adventuring party it's dead easy (a 2nd-level spell or a Paladin) but for a boss monster hiding out in a lair they may not have a reliable means of clearing this quickly.

Plus even if casting through the familiar triggers initiative rolls, it means you have a head start on the disease hopefully taking effect so instead of maybe triggering in the third round when the fight is probably already mostly over, it might trigger in the first or second and majorly turn the tide in your favour.