Decent
The game itself is decent. I like the idea, I like the control setup, and I like the difficulty. My issues are with the coding itself.
a) Controls. Multiple times I found myself cursing because either I would: click in a mob and drag to infect something, and none of my viruses were on my pointer, or I'd click directly on top of one virus, and another that was miles away would for some reason take priority. That's a SERIOUS issue in a game like this that requires quick thinking, because then I have to try over and over if, say, it's to save one from the edge, or to attack in a particular order to avoid a WBC.
b) Objects. It took me about 30 minutes to beat stage 15. Only because I'd get to 40-something spawned viruses, then hit a wave where not a single RBC came down. By the time one had come down, I was back down to between 10 and 25 on the screen. Random is good, but at that level it just leads people to get the bad kind of frustrated.
c) Upgrades. In particular, the immunity. When you infect a cell with a virus that's immune to anti-biotics (well. . . all viruses are technically immuned to anti-biotics, because they're not bacteria, but that's neither here nor there), at LEAST 1 that's immune should come out. Mutated strands of one thing or another that are immune to treatment don't lose that when they replicate. And if you're down to a few spawns, and the entire screen is covered in that fog, and WBCs are coming for your ass, you need to infect something. You do, and not a single immune spawn comes out, and you're dead. No way around it. So absolute bare minimum, 1 immune should come from 1 immune. Ideally, 2. I know more than that is a little ridiculous for a guarantee, for the sake of keeping a challenge, but 2 isn't going to break the game.
d) The graphics. There's not a whole lot going on, but it's still extremely easy to cause it to lag. I'm not entirely sure how the game was coded on your end, but I know the graphics themselves aren't enough to cause my computer to have trouble.
I won't go into the minutia, because then I'm just going into opinion and preference, but these big 4 are things that actually hurt playability. Solid concept, and even as is I played it for over an hour. If you got these bugs fixed, you could easily market it for android and iOS and actually make some money for your trouble. As is, though, playing it for free was a bit agitating.