There are specific combinations that unlock symptom combos in Plague, INC. I was playing as a virus (which mutates uncontrollably) and I had a ton of points to apply after infecting everyone. When I picked the path for public defecation (requirements), I did not receive the notification that the combo unlocked (nor the achievement) before everyone died.
Is there a specific path or time that these symptoms need to be unlocked with ?
I took the unlocking path of going from the bottom to the top:
(From Insomnia, upward to Diarrhea)
Best Answer
From the Plague Inc Wiki
Unfortunately, there is no mention as to how long a symptom combo takes to become effective, if this duration of time varies for each combo, or even if this value is constant. Continued research yields little results as well.
I would say it is very likely that you killed everyone before the proper period of time expired for the symptom combo to become effective (or there may be other criteria, there is very little on how to achieve a symptom combo besides what symptoms are required to trigger it).
To have the best chance to get the desired symptom combo, I recommend evolving the required symptoms as early as possible (after infection of everyone is almost an after thought) while avoiding lethality. This will allow you to accrue evolution points while you're waiting so you can kill everyone soon after you get your desired combo.
Edit: As per the comment by @Nolonar and some research of my own, these combos seem to be rewarded after a random amount of time.