There is a very effective, yet subtle underlying strategy effective for most if not all Plague Types in the game.
The Modified Genetic Code you want should most definately involve Ionized Helix, allowing you to refund Mutations later in the game.The other important Modifiction you want is the resistance to being cured. Getting bonus DNA Points for popping blue bubbles is also helpful.
The basic plan for this strategy is to run low-profile until you've infected at least 90% of the population. Some Plague Types however, are very subtle and can spread undected all the way untill 100% infection.
I find that a good starting Country is the UK, as it has a great multitude of air routes to the other continents, as well as a decent amount of water routes.
Your opening mutations should NOT be Symptoms, but Transmissions. The only Transmissions needed are the Air and Water ones, including the Extreme Bioaerosol mutation. While you work on those, it is recomended to begin Evolving the Drug Resistance branch to allow your Plague to spread easier in rich countries and to begin to build a preemptive Cure-Slowing.
If you're having trouble getting some of the Sahara countries or some colder countries, getting their respective Environmental Resistance (Cold or Heat), is recommended, because it not only allows infection of the country, but it speed infection of people within said country. (Historically, Canada and Greenland spread slowly, Morocco and West Africa are difficult to infect, and occassionally I even need to get the Environmental Hardening combination of Cold and Heat).
Pay attention to the unique Abilities of each Plague Type, as well.
For example, the Fungus Plague Type has its Spore Burst ability, as well as some very useful evolutions later on. Spore Burst is cheap, effective, and, in the long run, you can get some of your DNA Points back with the Ionized Helix. And if there's a country you just can't seem to infect, you can Devolve and the Re-Evolve Spore Burst, infecting the country.
All of this should happen very low-profile, with no symptoms. Any that evolve should be immediately Devolved, gaining DNA Points and staying undetected. If you are detected before 90% or 100%, it's alright, even if they begin working on a cure. After all, you have Drug Resistance and Genetic Hardening, right?
Once you get in the neighborhood of 70%-80% infection, it's okay to allow either the Nausea or the Insomnia Symptoms to evolve on their own.
Once you get 90%-100% infection, you can begin to get minor Symptoms of Nausea or Insomnia, as both lead to incredibley powerful Symptoms.
Nausea, in my opinion, is the best path to the devestating Total Organ Failure. Once you reach TOF, you can devolve Nausea and/or Vomiting if you wish, as that will refund you DNA points and you really only need the two fatal ones.
Insomnia is great for stopping and slowing the Cure progress, because everything in its tree that leads to Insanity causes behavioural abnormalities, and what Insane person do you know can Cure a virulent Plague?
Once you reach 99%-100% Infection, you can begin to devolve your five Transmissions to refund some DNA points to go towards things such as Genetic Reshuffle to set back the Cure. Always make sure that you keep at least Extreme Bioaerosol until you get the message saying there are no healthy people left in the world.
After all of this, the game is pretty much yours.