If you carry something, it will be on your hands clothes or shoes: have a pair of yard shoes that you dont wear out, wash up well after visiting other places, and change your clothes before you go to your animal area and you should be fine yourself, and I guess you could do a mini quarantine for hens you take out on the town.
Many diseases are just out there in the environment anyway, so it seems to me that keeping the chickens healthy is the best defense against anything...stress is probably as harmful to them and their immune systems as it is for us, so having a safe calm area for them is good (no predators). A dirty coop and run is unsanitary and probably stresses their systems. A balanced diet is essential. If the girls are handled and made accustomed to traveling they should have less stress.
Lots of posts about illness come from people who just know their birds enough and pay attention enough to notice when behavior is off, so attention is good, too!
Other than these things, there really isnt much you can do as a preventative, I'd think. I want to read the article on biosecurity...I know my sister did not like other farmers walking around in her animal areas. She said some people have a a wash up station with a shoe wash for visitors!
And good luck if you decide to show! Have a picture of your blue ribbon gal?
from Ohio. So glad you joined. Good advice given and if you study up on your poulty project the chances will be good. 
