Pretty simple achievement, to get it, do the following:
As a side note, no special gene modifiers are necessary pre-game.
1.) Start in any country you wish, I usually choose India/China due to high rates of infection.
2.) Evolve a couple of transmission types, such as Saliva I to get your infection rates up.
3.) Once you have accrued a decent amount of points (I was good at about 65) begin getting some symptoms. If you are also looking for an end game solution, wait until the globe is completely infected.
4.) Go down either wing of the symptoms until you hit the 'Cytopathic Reanimation' ability.
5.) Once the zombies start shuffling around, go to 'Abilities' and get the 'Horde Instinct' ability.
6.) Once you have that, from the main world view screen, press the left thumb stick to activate your ability (you should see an icon regarding it on the left side of your screen). From there select a starting country to have your horde start from and select a destination country, both by pressing the trigger.
7.) The horde rolls out, and soon, the achievement should pop.
Good luck infecting the world!