Can someone please explain the whole pox epidemic thing?

Well pox , back in the day was a sickness that didn't have a cure and spread fast , like a plague or something.In the Hunger Games I'm sure the capital has all the medisyn but the districts are to poor so they cant afford it.But I think the whole breed-pool thing is most of the bloodlines is full of sickness and they want new blood to have a healthyer next-genaration