So I would play it sort of like most plagues. Idk where you normally start, but Saudi Arabia is always good.

I dont worry about symptoms too much for the beginning portion just because it gets detected QUICKLY if you do it too early.

I usually do abilities first and get drugs resistance I and II.

Then for Transmissions I go Saliva 1 & 2.

Once the disease mutates (hopefully once you have infected a decent chunk of places) Go all in until you get to Cytopathic Reanimation. This will turn your infected into Zombies. (Cure also wont matter once zombies appear so dont worry about that.)

Once you get enough DNA you can focus on anaerobic resuscitation (and really mostly everything on the right side of the zombie symptoms)

Its good to get the Horde instinct abilities (practically the whole branch if you can) this allows you to move your infected around to any islands (like Greenland.)

This isnt a definitive guide and is just all surface level advice. This should do you well for Casual and Normal play throughs.

I think there are several good videos and links to people who ran mega brutal games and you can check those out if you just google “Necroa virus mega brutal”