I have a few questions. Was talking to someone in the chook world about buying fertile eggs, and they were very cautious about doing that. The concerns were that eggs can bring various diseases into your flock, so be very careful where you get your eggs from. Apparently just as dangerous as getting in sick chickens.
....Is this risky practice that has the potential for disease disaster, and therefore the best/ideal practice model? Or is everyones experience and reality very different?
Generally, hatching eggs should be less of a risk than live birds, if you are referring to the same source of stock. If there is illness in a flock - live birds are the best method to transfer the disease elsewhere.
However, having said that - there are a number of diseases that definitely can be vertically transmitted (ie. through the egg) including Mycoplasma spp. and Salmonella (eg. Salmonella pullorum). Generally, the rate of vertical transmission tends to be relatively low (although it can depend on what stage of infection the hen is at with the disease) eg. in many cases, say less than maybe 5% of eggs from a known infected bird will be infected (again depending on the exact disease organism, and the exact stage of infection). The problem, however, is that when that egg hatches, there the resultant chick will be infected and can potentiall spread to its hatch-mates (horizontal transmission).
The above refers to true vertical transmission (within the egg itself) - there is also the potential for surface contamination - which I think is what is being referred to in terms of cleaning eggs, and risk of transmission of black head.