I believe this requires a streak of 10 cures in a row. I got it on wave 3 of Topless Mountain. Make sure you have enough GPs offices (I had 3 at the time), a couple of pharmacies and keep an eye on the diseases as they pop up - build a ward or any other treatment rooms if needed. Also worth going to the policy (press X ->go to Overview -> RB to Policy), and setting Fast-Track Treatment Decision to on, (this will prevent patients being sent back to the GP for a final confirmation once their diagnosis is above the threshold on the slider, default is 90%, I set mine to 80%).
You can also abuse quick save and quick load if any of your patients either die or aren't cured.