The game is simple yet pleasing, the upgrades are your standard on this type of game and some were obviously more useful than others, for example virus turned out uless compared to something such as microbes.
I would advice possibly adding some upgardes to lead to move unquie paths, such as possibly a spread shot with weaker damage but larger range or a single powerful shot that took a while to recharge.
This game didn't offer anything new, however was entertaining for a while. Add a few more diverse viruses, some more variety in the upgrades and maybe increase the difficultly? I found the game not really increasing in difficultly compared to the power of my little cell.