I really like your game, very well thought, and overall very polished.
I just wish you could go back to previous visited areas. I found one exit before I finished discovering it. To be fair I also wish there a little bit more incentive on killing the viruses, some times I skip them because it didn't look necessary, but killing all gives some necessary power ups on some occasions. Having an extra incentive would be great.
There could be some improvements on the UI to be closer in the visual style of the game (plus, on a power up there is button label "Button" haha), and there could be more use of sound effects on attacks, death, power ups, it would improve the game feel, but overall that was a great game.