I think it was on another thread that I posted this, but when we were looking at houses to buy, we went to one that the lady had goats, and chickens. The chickens were free ranged, but there was a coop on the property. I talked about her chickens, but she seemed very reluctant so say much about them. I mentioned that I have Black Australorps that I show.
FINALLY she divulged that her flock had Coryza, and the vet had been out twice over the last few months to treat them. I was flabbergasted. Her neighbors had chickens, and they had expressed concerns too, asking her to please put them in enclosed runs so as not to contaminate their flocks. The nieghboring children were not allowed to go onto her property.
When it came time to look at the coop, we declined, since she had a couple hens in there undergoing treatment. No thanks! When we left, and came home, we took our clothes and shoes off in the garage, went straight in to take showers. The clothes, and shoes went into the washer, and along with the soap, I added a little bleach to disinfect them. Then I went out to totally disinfect the car. When I finished, once again, I washed my clothes, and took another shower. I was not about to contaminate my flock, if I could help it. All my efforts paid off, since my flock didn't get it. That's another reason I won't wear anything but "coop shoes" around my flock. I know she goes into the feed store to buy feed. That's one good reason I like where I get my feed, is that they go in the back to get the feed, and don't let people in there, so the bags are not as likely to get contaminated.
Yes, you are right about many of the animal swap meets being cesspools. I've been saying for years that there are more and more unscrupulous livestock breeders, and when they should be culling, they'll go sell sick livestock at the swaps, rather than take a loss on their sick animals. While the majority tend to be fine, all it takes is that one, or two exceptions to mess up entire flocks.