Start a Necroa Virus game on normal.
Activate:
ATP Boost
Creationist (most important)
Native Biome
Extremophile
Start your virus in the USA. Save up enough points for Saliva I and Drug Resistance I. After you have those 2 activated, you can sit back and wait for awhile. Build up your DNA until you have enough to activate Blood I, Air I, Saliva II, and Water I. This will speed up the transmission across the globe.
Then you just wait until everyone is infected. It might take awhile. During my playthrough, nothing mutated. But if it does, just devolve it right away.
Once everyone is infected, evolve Insomnia, Fever, Photophobia, Delirium, and Cytopathic Reanimation. With your leftover points, continue down the tree and evolve Anaerobic Resuscitation, Enhanced Motor Control, Peptide Surge, and Acidic Reflux.
At this point you'll probably be short DNA to purchase any further. But do not fear, soon you'll start getting the zombie bubbles to pop up and you'll get tons of DNA points from them. Spend any future DNA points you get on the 2 symptom trees branching right from "Peptide Surge" These traits basically ensure that Z-com won't destroy your virus.
Then just sit back and watch the world go down in flames! It will probably take you close to 2000 days to complete, but it's guaranteed to work.