You nailed it. Don't change anything. It is perfect as it is. I played for about 15 minutes. I am not very good at these type of game so I could not get past level 4.
That being said you did everything right. Nice menuing system. Nice intro to the game. I love the voice at the beginning of each level.
The speed of the white blood cell is good. The viruses dropping faster is good. It is much better than my game. So simple yet so very fun. Thanks for the great play!!!