Disease lowers attack, Crit, and healing. Burning lowers defense. Shocked and Freeze are strong as they are. But Poison and chill? Not that great. I have some quick suggestions:
Poison - Could lower Max Health every time it deals damage. As if it's corrosive or acidic. Would fit Phenol's theme of being a giant blob of acidic gel better.
Chill - Could lower evasion when applied. If you're chilly, you'll move slower. And Freeze makes it so attacks can't miss, so Chill should at least be partly like that, I think.
While this will probably be a less popular idea, I think disease is too strong. It should at most do 2 out of the 3 things it does. Negates Crits, lowers Attack, and lowers healing. Choose 2. 3 is just way too powerful. I believe it shouldn't lower Attack, because negating Crit and lowering healing are cool and unique side effects, while lowering Attack just seems like the cheap icing on the cheap cake. I understand disease is a good MK counter, but even just negating Crit, it would still be somewhat effective against him.
Perhaps we could put "lowers damage" on Poison. I don't know. I can't fight to my fullest when I'm feeling nauseated. Although I guess both disease and poison would cover nausea.