How about starting by adding health on a per city basis :
First, it will follow the same pattern as happiness with three different states (healthy, unhealthy and very unhealthy)
- healthy will work toward a counter that will trigger a "WLTKD" effect. I'm not set if it should stack with or extend time duration of the already existing WLTKD.
- unhealthy will work against the aforementioned counter and, in addition, could reduce growth, science and culture by a small percentage.
- very unhealthy will start at -10 and will multiply the effects of the unhealthy status as well as reducing the great person birth rate.
I think it will require some tweaks in other aspect such as resources, buildings, specialists ... etc ... but this is a general idea and a good starting point. If someone wants to dig further, I will be glad to discuss with him and design the health module this game desserve.