Awesome game mate, played 4-6 hours straight. Having an infinite upgrade path is nice, clean visuals, bug free as for as i played. Wish-listed.
Would love an instant animation option(for card animations, turn processing time, card effects, virus movement, etc.) and auto end turn when there are no viable cards or enemies.
Add tons of meaningful content, bit more polish, balance new cards yet keep their uniqueness and tighten difficulty. Don't be afraid to compact the game-play loop if needed; games of this genre can drag on or become tedious after x amount of turns.
If player keeps using shields and doesn't kill anything, you can survive infinite waves. Viruses don't spawn if it's spawn point is blocked. Either allow virus merging towards player when all tiles are blocked, or newly spawned virus values are added to the viruses that currently circle the player. Cheapest option would be to reduce player health to 0 if virus can't spawn.
Best of luck with the game.