I usually start in India or China.

Perks:
Metabolic Jump
Darwinist
Extremophile
Trans-Stasis
Aerocyte/Aquacyte (depends on scenario - if there are no planes pick Aquacyte)

If you're doing climate change scenarios pick a hot or cold country

• At first you have to evolve Virus Instability 1-3 to increase the chance of mutation.
  If you having trouble infecting the whole world you should evolve Bio-Aerosol first
  
  
• After you're done with Virus Instability you have to evolve all the different types of transmission, starting with the animal transmissions - further increasing the chance of random mutation (you must not devolve anything! It is not important if you get detected)
  Feel free to evolve the Cold Resistance and Drug Resistance ability to increase your transmission rate in colder and richer areas.
  
  
• When you're done with evolving transmissions you will already have plenty of symptoms in your arsenal thanks to the high rate of random mutation. Further envolve non-lethal symptoms to increase infectivity.
  
  
• When every country is infected and you're done with step 3 you can evolve more deadly symptoms (if they haven't mutated yet - you will have plenty of DNA left anyway)
  
  
• Spend your DNA to become deadlier or decrease research progress (they won't get far)

Congrats you successfully managed to beat your scenario of choice!