You can unlock a number of evolutions in the game, each of which will give your disease various traits that make it spread faster, kill off humanity more rapidly, and become more resistant to cures. Out of all of the evolutions, the only one that can’t be fully unlocked in easy mode is the Cysts one. Cysts level 1 CAN be unlocked in easy mode, but level 2 and 3 require you to win in normal and hard mode.
Other than those two, you can unlock every single one of them in easy mode, so do that in order to unlock them faster, with less effort spent, and in fewer games than you would normally have to in normal and hard mode.
Unlocking every pathogen is a little bit more time consuming, though it’s still a fairly quick process. All that you have to do is win in normal or hard mode while using the previous unlocked pathogen. This means that you can only unlock one single pathogen at a time, but it’s fairly easy to do – just repeat what you did before, and you’ll win easily every single time.
Pay attention to the news popups that appear during the game. While they often seem silly, many of them have implications as to how you should play and what new disease traits you should purchase. For example, if it’s unusually heavy rain, then bugs will flourish, as well as waterborne transmissions. If it’s unusually dry, mosquitoes and water will be less likely to carry it over, but migrants might be more likely to transmit the disease.
However, all of the news briefs that scroll across the ticker at the top of the screen are completely unnecessary, and are entirely fluff. You can ignore those, although they are fun to read for entertainment, since they’re usually extremely humorous.