I am in endless mode, I found if I do the plague when there is herbs, and advanced meds, they prioritize picking up items off the ground and building, instead of getting herbs. which makes the plague stuck in the world as they never cure it, and they die before they can do what they are trying to do
For plague cure you also need Venom (extracted from snake which in turn is spawned by playing critter on jungle tiles). But you are still correct, unless the situation is really stable in terms of food and buildings seems like they usually don't prioritize developing the vaccine. But in endless should just be a matter of spawning more humans until they finally do the vaccine.
