First, the most important thing is to devolve ANY symptoms except for Coughing the second the window pops up. Coughing is the only symptom that will not alert the world to your disease. The point of this strategy is to slowly and quietly infect the entire world before you add deadly symptoms all at once.
I start the disease in Greenland, and pop any bubbles that appear. The reason is that infecting the cold climate countries when starting in warming nations can be very difficult, but when you start there it spreads very easily. I had several games where I killed everyone on Earth except for Greenland or Iceland before switching to starting here.
As soon as I can, I buy the Water transmission level 1 & 2. By Ship is the only transmission vector out of Greenland, but this will lead to Iceland, Norway, and Russia, which will then spread to many more nations.
Then the Drug Resistance ability as the first countries that it spreads to from here are advanced and will slow the spread otherwise. Plus you'll need that anyway to move into the Genetic Hardening abilities later.
Then Air transmission 1 & 2. This will radically increase the spread of the disease. to the southern half of the world, especially once England gets infected through Iceland.
Africa tends to be the problem child in this strategy, but adding Blood transmission 1 helps, as can the Heat ability to spread more easily in hot climates. In this strategy Morocco and Zimbabwe tend to get fully infected last for some reason.
After Blood transmission get Genetic Hardening ability 1 & 2, and speed up time. This will greatly slow the work on the Cure once you pop the cork on symptoms. Continue to devolve any symptoms that randomly mutate, earn as many DNA points as you can popping the little bubbles, and wait for the disease to spread.
Once EVERY PERSON is infected, SAVE. You should have enough DNA points to follow at least one chain of symptoms to the most fatal. Organ Failure through the Coughing chain is very effective, but all of them can lead to very nasty ways for people to die. Also, you can pick up a few extra points by devolving all of your transmission methods since everyone on Earth is already infected.
As people die you will start gaining more points. Try to save enough for a Genetic Scramble ability to slow the Cure if its progressing too fast. If the Cure isn't keeping up, then have some fun and add more deadly symptoms to speed the death of the world.
This strat works on all diseases until Nano-Virus, which requires a completely different strategy, and it works even on Brutal difficulty. The hardest one using this strat is Virus on Brutal as devolving symptoms costs DNA points rather than adding points back, but its doable.
The secret to life is honesty and fair dealing. If you can fake that, you've got it made.
Coughing + Sneezing better for spreading infection. Doesn't start research for cure. Beat all on Brutal by starting off in China and never really had to much trouble with the cold places except Canada but thats because it was rich and cold and never really worried with getting the drug resistance 2.