Sorry for the long text. Also exuse me for all potential spelling mistakes
Hello, your mod is excellent and after playing it, i think some features cool could be added.

So here some suggestions :
-Three type of disease : the small ones, mediums and big.

-Small disease : small disease like a throat infection are not mortal because our body know how to defend itself agains't it. But in the past, human body didn't know this disease and couln't defend. The idea is that the first wave of this disease would be mortal, the second a little less etc until it do nothing to your population. They will have one disease of this type per continent. That mean that if you are european and you meet a civilization from another continent (like Americas), the other civilization will die of your small disease (like IRL).

-Mediums disease : mediums disease are limited in number (like 4) but will reappear multiple time in the game. Unit like The Plague Doctor can fight them but the best thing to do is to vaccine the population.

-Vaccine : Unlocked with the "Sanitation" tech, the vaccine is a city project that stop the spread and effect of ONE medium or big disease in THIS city. It will also immune all units of this disease the first time the project is finished. A big disease will cost more production than a medium one. Also, a disease studied by a Scholar could lower the production cost of the vaccine.

-Big disease : also limited in number, big disease appear once but have specific features like : Quicker spreading (2 tiles instead of one), deadlier or longer effect. A big disease appearing could also create an Emergency at the world congress and vaccine your population will grant you point.

-Unit spreading : when a unit is in a infected tile, it has a chance to be infected and spread the disease. That should be how a Trader can spread the disease.

-Disease starting : The start of plague by starving and negative housing is a good idea but with a trader is not. This unit literally become a weapon and should not. So as a replacement i suggest that animal became a source of disease. So if in your territory, you exploit one animal related resources, it could be the start point of a disease. They are : Horses, Cow (or Cattle), Sheep, Deer, Elephant (or Ivory) and Fox (or Furs). Also, small disease can only spawn on thoose tiles.

-Great People : with the Vaccine mechanic, adding Louis Pasteur as a great scientific could be great. Plus a tech bonus to get "Sanitation", it could add a +100% production for vaccine project.

And that's all for me. Only shame about this is that it kill the usefullness of the Scholar.
I know that some suggestion could be hard to add and you can just ingore this post, I would still be happy that you read it.

Bonne chance pour ton mod ;)

Oh, someone has discovered I am a French speaker despite all my efforts... Damn my accent must have betrayed me again! :p