I found it very fun.
You could improve the ratio popups get created. And are the math questions randomly generated or always the same?
Also, when I finished all math questions there is nothing to do and the Virus still took over haha.
The progression could be attached to some world entity (like how many popups are the + how much time it was there), it looks you are using time only.
It would not be that difficult to randomize and dynamically generating popups making the game endless (some score system could be needed if that was the case).