A guide written by Tatsur0 here:
http://steamcommunity.com/sharedfiles/filedetails/?id=288653...Goes over the best way to do this achievement. I have now tried and can confirm this method works. If you decide to use this, please do at least give him a thumbs up for a job well done!
So to give you the vast summary of his guide, you start as bacteria and your starting country is Saudi Arabia.
The genetic code you choose for your bacteria will be ATP Boost, Genetic Mimic, Aquacyte, Urbophile and Sympto-Stasis.
Step 1 - Symptoms: Rash, Sweating, Skin Lesions, Coughing, Pneumonia, Insomnia and Paranoia.
Step 2 - Transmission: Air 1, Water 1, Air 2, Water 2, Extreme Bio-Aerosol and Rodent 1.
Step 3 - Abilities: Cold Resistance 1 and 2, Drug Resistance 1 and 2 then Heat Resistance 1.
Step 4 - Symptoms: Seizures, Necrosis, Pulmonary Fibrosis, Total Organ Failure, Insanity, Coma, Paralysis and Dysentery.
Step 5 - Abilities: Genetic Hardening 1 and 2, Genetic Reshuffle 1, 2 and 3.
After step 5 you should have a vast majority of the world infected and dying. Once more, all credit goes to Tatsur0 and the one contrib that made the guide. Ensure you give them the full credit they deserve!