Cover name, creator, system, jam, and an idea of what we can expect. At a glance of everything as a whole, seems pretty obvious that everything ties together well. As a whole, when I take the work to the scale of what it would print at, it's a bit small on font for my liking. On the up side, the stark black on white element means I won't have to be worried about contrast issues from a less than amazing printer rendering it into a bunch of unreadable fuzz.
Tracker along the bottom is a nice setup. Keeps with the visuals and creates an easy way to follow what's happening. The only down side I can think of is when needing to access information from the other side. Then again, switching from laying a token on it to a paperclip instead probably works just as well. Trigger section kind of frustrates me. It's interesting, but there's only a 1:6 chance of infection. Since I can assume immediately the infection is vital to the story, this is an issue for me. A few lucky rolls and no one is infected. I think the easiest solution for this is to make 1 item automatically infectious, while all others have the chance. This way you are sure to have one infection, but potentially several.
Progression is simple enough. I'll have to see how the game is meant to play to know if some groups might screw this up somehow or not, but it's probably fine. Treatment catches me though. It says explicitly that after stage 1 it is incurable, but then goes on to talk about advanced reversals for stage 4. I think what you're aiming at here is the disease becomes a chronic and permanent thing the player has to deal with after stage 3, but that stage 4 and above can only be dealt with through advanced methods to prevent full change. So a lifetime of dealing with it through expensive and difficult methods if they allow it to progress too far.
Then I look back at the rumor table and realize the 'halt' element comes up as something more than it initially seemed. Having thought it through some more, it seems what you are trying to say here is that you can reverse the 'progress' section, but any 'mutations' will remain even afterwards. I think if you are going to focus anywhere, this is where to put your attention. Adding clarity to the way this is written would go a long way.
Rumor table seems fine. Good choice on the True/False element. I started doing that with my own and it makes it a lot clearer to the GM how to handle their rumors when they happen. I've taken to enjoying adding a half-truth in there as well when space allows for explaining it. On progress, I like how the early stages are beneficial, making some players think they can hang on to it a bit. It's a strong method to coax them in to allowing it to get past stage 1. However, I think I would make the 3rd a trade-off stage. Not just a negative, but a positive and negative together. This is the hint to players they may be getting the worse end of the deal if they let it go on. It's storytelling through the mechanics and lets the ones who decided to 'let it go a little longer' before curing it look back and realize exactly when they made the fatal error. One final element is that 11 is basically death anyway. I suppose there's a chance one of the other players undoes the effect by dropping you down a few notches somehow, but since the player in question won't be helping them at that point, that player is out of the game for a while. I'd probably have them lose memories and consider the priests favorably, but let them still interact with the group rather than having them /only/ respond to priests.
Roster of enemies is pretty clear. The only thing that threw me was the infested priests. I assumed the good father and his lot have plant-associated powers due to their faith. Apparently being of the faith and infected means you don't turn full shrub, but get nettle fingers. Makes me wonder if the players can joint the faith to avoid permadeath.
Mutations are interesting. I could see 3 having a mechanic attached. 4 having a risk of infecting the other players, etc. Mutation 10 overlaps with Progress 9, which could make a weird overlap of effects. That last one with the mushrooms I would love to have fruiting 1/day with edible fungi that count as a ration if one of the other players is willing to do so.
Last page of the single page format puts everything together. Why they are here, what they are doing, etc. It's the first entry I have read that wasn't an adventure, or map, or similar attempt at a larger scope. Instead, you've focused more tightly in on an element and allowed it to sit freely to be placed into an adventure of the GM's choice. And honestly, it works. I feel like I have everything I would need to plug these guys into my own campaigns. I don't play Cairn or Into the Odd, but I could still make these work with minimal effort on the games I do play. Several of them in fact. It would be so simple to add hints of their existence months before they show up, building the stakes in advance and allowing players to know full well they shouldn't allow this to progress.
I think you've made something very solid here. If you decide to expand, I am going to make an odd suggestion. Instead of expanding this, just clean it up and keep it either free or low cost. Do an expansion that is a full adventure set around using these guys in part of a larger setup. It lets people see the quality of what you have made and I think as long as the price was reasonable, it is a solid incentive to purchase whatever future expansion is associated with it. Just my thoughts though. Good work.