Really cool game! Maybe I'm a little dense but admittedly I was kinda just guessing by the later levels. I also couldn't get past level 15. I would just run the simulation and watch how it played out and try to adjust my walls and infections accordingly.
One thing that I think might have helped me internalize how the game worked would have been the ability to pause and "step" the growth of the parasites. I was sort of able to achieve this by turning the speed slider all the way down and then moving it up and back down to make them advance a cycle. This is something I learned with my game--when you've made it, you're very accustomed to how it works so it is a lot easier for you to understand how a level might play out. Next time I'll try to have more friends playtest so I can get a better feel for how a noob experiences the game.
Nevertheless, everything game together well into a nice puzzle game, great job!