Cool
This game is really cool, i really like the idea, and the sound u got there suits the game very well. :)
Anyway, you could add in more stuff to increase the support the whole "end of mankind" feel, like instead of just message u could show newspaper with pictures of families crying, and scientific study of the virus, later, mass graves of people, and the deserted streets. Also you could make, some screen in the update section in which the virus cell would be shown moving around, and multiplying or stuff, and it would look different depending on the upgrades.
So overall: The game is really good, but there's hell a lot of stuff you can still implement to make it even better.