The Plague is underpowered and here's why:
I played more than 10 games against and as The Plague and as a survivor main with 2500+ hours I can tell she is really underpowered.
This is mainly due to 3 reasons:
1) Being poisoned doesn't hinder survivors juke potential. I literally stayed sick the entire time of 3 games and it was no problem. Competent survivors usually don't spend time healing anyway (this is the same reason why legion is weak, survivors just don't heal against him) and stay injured to gen rush even more, especially with dead hard and adrenaline. Being broken also doesn't do anything because it doesn't matter if you are injured with or without it.
2) Survivors can deny the killer getting Corrupt Purge (damage with puke when interacting with a used pool of devotion). As stated in 1), survivors can just decide to not cleanse their disease and stay injured. The Plague will have no way of getting corrupt purge to actually work if survivors decide to do that which will be meta against her for sure. So she will have even more hard time against good survivors.
3) Survivors can FULLY HEAL when drinking water. This is just stupid. If you affect a survivor with the disease and you injure them they can fully heal in about 5 seconds. Sometimes when I was already injured and not infected I just intentionally touched an infected objective and then healed really fast at a fountain. So if you injure survivors and switch targets it actually benefits survivors because they can safely heal if they decide to.
How to balance her without making her brokenly OP?
My 2 simple and obvious suggestions: Firstly, being infected or broken should reduce your action speed (repair/heal) by about 30% so survivors basically HAVE to go and use a pool of devotion.
Secondly, drinking water from the pool of devotion should not heal survivors but only cleanse the infection. Maybe increase the time it takes to drink the water a bit.
In addition to that on a "lower priority level or call it QoL" I also wish survivor's location or aura would be shown briefly when they puke because right now you don't really recognize that unless they are in front of you. This would also increase the survivor's incentive to cleanse the infection. As suggested by other users I also think some addons could increase the range and velocity of the puke but mostly her addons are fine. It also feels kinda weird that you are slowed down so much/long after releasing your puke. I understand why it is how it is but it just feels weird and a bit unfun to be slowed down so much.
Additionally, the whispering when in corrupt purge should be quieter. It's too loud and a bit annoying in my opinion.
I know it's early feedback but I can absolutely see those problems I stated becoming really bad especially when survivors start adapting to her even more and realising that being poisoned doesn't really do much.
I hope the devs take feedback of the community to balance her right! She is such a cool and well designed character and I hope the PTB and this feedback will help the devs to make some good adjustments for her.