There are several diseases that can be carried within the egg. MG and MS to name 2. I believe it is a reportable disease in the USA but not in Canada for chickens (it is for turkeys) MG is a serious threat to commercial egg producers.
About a year ago, maybe a year and half, I received notification from a member on a Canadian Forum that I am Admin on. Seems that "the powers that be" have been monitoring forums such as this one for information on people bringing eggs over the boarder into Canada. Some folks just claimed them...
pour on Ivermectin works very well. put a drop on nape of neck. A product that we have used with great success is Bronco. It's actually a horse product. Once spray under each wing and one spray on the vent area.
Yes, very easily mites when it's this cold. They attach themselves to the chickens and keep warm and reproduce on the birds. They have a nice warm host as shelter and nice warm blood to eat. A lot of people think that as the weather gets cold, the mites go away. Not so.