As far as I know, no other 4X game has somthing like that .. maybe the plague in some Paradox games - dont know that. 

This situation shows how fragile our comminites and countries can be. 

A mechanic/event with plagues/pandemics would be cool ... 

Have it exist as a city project that takes X turns to build, and is the only thing the city can build while diseased.

Before vaccination tech, a disease will kill 1/3 of the pops, and take (remaining_city_pops) turns to build.

Before genetic sequencing tech, a disease will kill 1 pop and take 2 turns to build.

Afterwards, the disease will not kill any pops, but the project must be completed to remove the disease.

I would also recommend looking at Civ 6's scenario "Black Death." It may be too detailed for the entire game, but I found it was pretty good historically and gameplay wise.

There was an apothecary building in the tech tree from one of the feature focus videos, so maybe there is a health mechanic of some sort.

I think if somekind of "plague and disease" it should introduce smaller disease that just give you small debuff and plague that can change the entire game (black plague on middle age in Europ that kill a third of the population of the continent). Of course, you should also have the possibility to voluntarly give the disease to another player (send an infected into a city or chimical weapons on modern era).

I think there must be plagues that are spreaded by trade routes and must be decisions to stop it or prevent like closed borders or blocked trade routes (and vaccination and quaranteen of course). Of course decisions must affect the economy.

So biological wearpon there must be also with apropriate technologies.

This would be an excellent add-on later. I love the idea that the plague or disease could spread along trade routes. It would add a strong "self-reliance" angle to the game.