Australia has created a very high-tech nano-virus to help the immune system, but couldn't be released because doctors said that it was "too unstable". So people stopped paying attention to it, even when it managed to escape from the lab and infected patient zero.
Since the nano bot is high tech, it starts off with drug resistance 1/2 and drug immunity (the nano-virus exclusive trait that made it invinvible to all drugs). The nano bot should also start with heat resistance to protect from Australia's typical climate.
The nano bot starts off with crappy symptoms unless you evolve respitory shift, circulatory shift etc... A new tier 2/3 symptom could be "Immune Hijack" where the nano bot hijacks lymphocytes to attack itself which is severe and slightly lethal. A new symptom only with the circulatory shift could be "Anaemic blood" where the nano bot destroys red blood cells to cause hypoxia - severe but not lethal.
If you guys have any more ideas you can add them here :)