Critics who treat adult as a term of approval, instead of as a merely descriptive term, cannot be adult themselves. To be concerned about being grown up, to admire the grown up because it is grown up, to blush at the suspicion of being childish; these things are the marks of childhood and adolescence.
- C.S. Lewis
Also, Zonapha Syndrome from Karakuri Circus is pretty nasty:
Stage One
The most basic stage. As disclosed above, at this point the infectee will have seizure attacks at random times where their trachea contracts, that will cease when the people nearby will laugh for them, thus causing a relaxation on the infectee's system. This is only a temporal solution and will not prevent people from reaching the next stage.
Stage Two
At this point infectees have their mobility reduced and the attacks are more severe. There is an increased chance of death and most infectees at this point, actually succumb to the sickness and pass away. On this stage, laughing is hardly effective on infectees for the sickness has developed beyond what a mere laughter can remedy. Death on Stage Two although crude is by far and lot considered a blessing, for the true terror of Zonapha lies on those unfortunate enough to reach the last stage.
Stage Three
The final and most terrible of all the Zonapha stages. People who have reached this stage are left on an absolutely catatonic state without any possibility to move or communicate through any means. Seizures have stopped, but there is a catch, people on this stage cannot die, as such they are doomed to suffer for an eternity as they are fundamentally dead on earth.
"Do you know what the most poisonous gas in the world is?"
There's "most powerful" and there's "most interesting", and this thread has more of the latter, which is hardly a bad thing.
With that said though, poisons and infections are really quite different so:
Infections: Someone already mentioned Marvel Zombies, but I'm seconding that. It's quite hard to beat in terms of the sheer power of entities that can be affected. However, Marvel zombies aren't all that debilitating, so..eh.
Poison: I nominate real world poisonous substances that can kill you before you know it. Yes, I am referring to stuff like CO, ((CH3)2Hg), and the like.
Came into this thread expecting to see this. Was not disappointed.
On the note of instantly flesh melting poisons, for poisons, there's Megellan and his Doku Doku no Mi, which when he was punched a couple of times by a class 90 durability supersonic speedster, caused the said puncher to go blind and have their hands start melting. That's just from a couple of supersonic punches. From a dude that otherwise shrugs off being smacked across cities, and stomped on with island shattering strength.
Magellan was pretty bonkers.
Why are we here?
"Superboy Prime (the yelling guy if he needs clarification)..." - Postmania
"...dropping an orca whale made of fire on your enemies is a pretty strong opening move." - Nik
"Why throw punches when you can be making everyone around you sterile mutant corpses?" - Pendaran, regarding Dr. Fate
As far as actual toxic substances go, His Tears may not be the best feated in terms of affecting anything more than humans, but goddamn if the effect isn't completely horrifying.
I am a mighty wizard from magic lands
also that plague that nurgle created solely to troll gulliman.
This is more venomous rather than poisonous, but the new(ish) Abomination from Immortal Hulk has a sort of gamma vomit that can eat through the Hulk pretty easily.
still surpised nobody mentioned Nurgle's Rot
In a similar note, jaki/miasma from Inu Yasha, especially Naraku's.
Normal miasma is a poisonous aura that can melt people or Stone just from being in the general vicinity. Naraku's aura can instantly kill/melt entire forests when he's not deliberately hiding it, and even caused powerful monks like Miroku to have to occasionally stop and have to vomit from just being in the same mountain as him. Plus, Naraku's able to generate concentrated bursts of the stuff that melt mountains and almost insta-kill people who are immune to his basic aura.
And Sesshoumaru, in addition to having potent poison of his own, gained Bakusaiga, a sword whose cuts spread like poison. That is to say, if it can so much as scratch you, that scratch will spread though your body in bursts of intensely powerful golden lightning until it has destroyed you completely.
From One Piece we have the "mummy virus". You get shot with a bullet containing the virus, and then you get an incredibly hot fever and gushing blood until you whither like a 'mummy'. And once one person is infected, anyone who touches/is touched by them is also immediately infected.
Critics who treat adult as a term of approval, instead of as a merely descriptive term, cannot be adult themselves. To be concerned about being grown up, to admire the grown up because it is grown up, to blush at the suspicion of being childish; these things are the marks of childhood and adolescence.
- C.S. Lewis