I agree. I think everything will pivot on a new “health” metric. You should start out fine as long as you have fresh water and multiple food sources. Droughts and famines could weaken your civ’s health and make it susceptible to plague. On the other hand, a rapidly growing high pop density city w/ no aqueduct or sewers should be a filthy mess burgeoning with disease, also leading to plague.
Full blown plagues should be as rare as volcanic eruptions, unless you haven’t been taking care of your people.
Maintain the balance with proper sanitation: the new “medicinal herbs” resource can help in the ancient era, as can the plague doctor unit and hospital building in the Middle Ages, and new pasteurization and vaccination techs in the industrial/modern era, which give a health bonus Civ-wide when researched.
Disease can run rife in any era given the right conditions, whether the Black Death in the Middle Ages or epidemics in the embattled trenches of WWI.
I suggested in another Thread a 'Balanced Diet' mechanism for food-based amenities, but the same thing could be applied to a Health metric even better:
Basically, food Resources are divided into three groups:
Meat/Protein: Cattle, Sheep, Fish, Crab, Deer
'Bread': Wheat, Rice (a good excuse to add Potato and Maize Resources here)
'Flavor': Spices, Bananas, Citrus, Cinnamon, Cloves
Having just 1 Group represented would give you a relatively low level of health/Amenity. 2 would be better, having all 3 would give your population a high level of Health and disease resistance.
Introducing this would almost demand that we also introduce Trading of Bonus Resources as well as Strategic or Amenity Resources which I believe would not be a Bad Thing at all: Trade is still somewhat under-emphasized compared to its historical importance.
Amount of Food would also contribute or detract from 'Health'. If you are maxed out, a Drought Disaster could turn into a Health disaster as lack of food weakens the population and brings on an Epidemic.
There are a number of Buildings and 'civic situations' from Ancient Era on that could also contribute to Health levels:
Baths, Thermal Baths
Mill and Granary, both representing more efficient processing and distribution of Food
'Clean' Water, represented by access to river or oasis or lake water or an Aqueduct.
Sewers
Techs like Sanitation, Chemistry, or Scientific Method could also have a Health modifier attached to them - or Policy Cards could be available with those Techs to introduce a potential Health Component.