This is such great work. I think the "collage" art style is done so well here, and all the animations for humans turning are great.
I do think the whole game logic revolving transmitting the "parasite" is a great design concept, and I felt like I was losing control pretty fast even on normal difficulty when more people were infected than not. Speaking of difficulty, nice replayability with the different difficulty settings.
One thing would be I wasn't really sure what to do with the parasites themselves, and what to do after all the walls are cleared. I thought the little indicators were a great addition, but I had no idea what they were actually pointing to for the first minute. I think this is a very small problem though and could just be me being dumb. Anyway, this is absolutely fantastic stuff!!!