Well, I'm not sure HOW I got this, but I got it on the Science Denial scenario.
I was playing with a Bacteria and did a subdued playthrough, making sure almost everyone had been infected before going in for the kill.
I started over at Anaemia and went up towards The central kill symptom. The disease was cured, but as it was being dispensed, the riot happened.
Not the best help, but I hope it gives you at least a vague pointer.