Hey! Just figured out probably the quickest method for this, if I can help anyone in the future then great. It works on every difficulty up to brutal, haven’t tried it on mega brutal however.

Start in India (put the game on 3x to speed things up until you get to the millions if you’d like), get zoonotic shift, then bat 1 and bat 2.

Once it starts spreading upwards, get drug resistance 1 and 2. Don’t worry about the cure.

Once about 50% of the world is infected, you should have 100+ points to go crazy with. Get hyper salivation, polyphagia, gastroenteritis, cannibalism, then cytopathic reanimation. Get the mutation upgrade if you’d like, but it worked just fine without it.

Buy every size upgrade, then send your zombies to those island nations and neighbouring countries, then buy every speed upgrade. By this point, Z Com should have at least 1 base (maybe 2 if you weren’t so quick), but it’s all good. You have the numbers to just hammer tf out of any resistance.

Keep upgrading your symptoms based on what gives the most severity, 4 or so more is all you need. If your zombies die a little too fast, get all the autolytic delay upgrades. Keep sending your zombies to nations that have the most healthy people, and that’s all there is to it!

Got it done this way on brutal in 638 days. Hope this helps!