On my first run I had no idea what to do. I didn't know you could shoot and I just let the virus thing take up 90% of my screen because though it would just go away. Then I found the tutorial... I think I must have skipped it some how.
Anyways, the difficulty was just right. I like the virus mini game where you have to shoot all of it before it spreads. The control scheme is pretty bad. There is no comfortable button to shoot when using wasd. I used arrow keys, z and space in the end. I didn't use the movement slow button because it's too far away, but it's a nice feature to have. An auto fire for single player would be nice instead of perma holding the fire button. Some UI problems already mentioned in previous comments making it unreadable. The bombs are kind of overpowered, destroying all projectiles, enemies and viruses. Overall, it was very fun!