It gets a bit difficult to do that though in later games with differing plague types that behave differently from one another.
Viruses mutate quickly and cost DNA to devolve, so you could end up with symptoms that give your plague away faster than you have the world infected.
If you play Plague Inc. on mobile, there's quite a few plague types to add some variance to gameplay.
Bacteria, Viruses, Fungi, Prion, Bioweapons, Nanomachines, the Necroa virus that creates zombies, the Neurax Worm parasite that can enslave those it infects, and the newest one is a disease like Planet of the Apes. Makes apes intelligent, can kill humans.