At higher difficulties you'll get discovered before you can get in every country with minimal infection and zero symptoms, and one of the criteria for scoring is how fast you can win so if you want to get max score on the hardest difficulty (which you need to unlock the special plagues without paying) you need to be aggressive. I unlocked everything except the vampire plague when i played this years ago, because the Planet of the Apes scenario is really hard on max difficulty (I hated that one because the point of the movies wasn't to make humanity go extinct, just replace them with apes as the dominant lifeform on Earth)