First, I'm sorry you have to deal with that. It surprises me that it was the guineas. They are usually more disease resistant, or so I thought. Someone I knew once joked that you can't kill a guinea with a .22, meaning disease bullet-proof.
I have always free ranged my flocks, always. That's over 12 years now. I don't put out bird feeders to get wild birds to hang around, but every year, phoebes build nests on my house and wrens sometimes build theirs on the old coop in the back. So far, no illness that I know of was in any of those wild birds. Generally, I think they go off and die (probably in those big chicken warehouse operations, LOL)
The guineas, sadly, were asymptomatic carriers - they themselves never showed any signs of the disease, but they had no problem infecting my chickens with it. I'm just glad that of the diseases, it was coryza, because coryza doesn't affect waterfowl, thank goodness.