The Ghost Plague is a disease in Settlers 6 that first appears in Geth. The first time your settlers will be affected by it is in Juahar during a scripted event where you need to seek out the cure at a village.
The disease is very crippling to large cities as it makes your settlers go on strike. Only manufacturers are affected as resource gatherers only require the food need to be filled. However eventually all your manufacturers will go on strike if there is no medicine and there will be no one to produce food or earn income. There will be no tax to collect and your army will fight poorly due to the lack of gold to pay them, and the resource gatherers will go on strike due to a lack of food.
Usually the ghost plague strikes when you have around 200 settlers or during a scripted event. Alandra's active ability heals settlers around her for a few seconds but it takes some time to recharge. It is best to build a few Herb Gatherer's Huts before achieving the title of Marquis or befor reaching 150 settlers if there is enough space in the storehouse to store herbs.
If the herb gatherers strike due to a lack of food is it best to load a previously saved game or buy food from a village as there will probably be no food producers that are not striking. This in turn can eventually cause the entire settlement to strike either due to sickness, hunger, or a lack of a good.