The amount of people that spawn seems to be buggy. The first few rounds after you load the game all have 70+ people. If you continue to play, however, the number drops significantly. I just got to a round where I needed to infect 40 people, but there were only 37 on the screen!
It can be fixed by reloading the game each time your turn ends, but that's pretty annoying since this game takes forever to load.