Consider a scenario in which your group is doing absurdly well. Three diseases have been eradicated, and while the fourth disease — Red — has not yet been cured, it has been contained to a single city, in which the Medic is already present.
Then it happens. The Operations Expert, holding three red cards, moves to a Research Center where the Researcher, holding two red cards, is already present. The Operations Expert takes those two red cards and then cures the final disease. You've won!
But since the Medic is in the only city still infected by the (now cured) Red Fever, that one is instantly treated as well.
Have you now won with full eradication? Or did the game already end once the fourth disease was cured?