This is from a relevent government site.
Although the NPIP has several classifications related to freedom from various diseases, the only required classification for Plan participation is U.S. PullorumTyphoid Clean.
Each State can have its own requirements for NPIP and different shows can have additional requirements for testing and vaccinations, but the basic NPIP is pretty limited. It has been extremely effective in taking something that used to be pretty common to something that is now pretty rare, but don't count on NPIP to protect you from any other diseases than the ones it is designed to protect you against.
I have also seen an obviously sick chicken at a county fair show. I was amazed that people that know chickens well enough to breed some pretty nice chickens didn't isolate her. And I know a specific person that lost a "championship" line because of bringing home a disease from a show, or so they said.
Quarantine done the right way at the right time is a great idea. I can't argue with proper quarantine at the right time at all, especially if your chickens are really valuable to you. I'm not worried about mites, lice or worms. Those are just a nuisance as long as you don't let them get out of control.
The concern is that you might bring in something that can kill your flock. When it happens, it is really bad, but it does not happen often. Many people bring home chickens from totally uncontrolled chicken swaps where they are exposed to many different chickens from different flocks and never have a problem. You'd think that chicken shows would be even safer, not so much from NPIP requirements but mainly because you'd expect the people there to know enough about chicknes to recognize a sick chicken when they see one. Of course all you need is one person that "knows what they are doing and doesn't need any interference" to mess anything up.
From my experience, most Oldtimers are not going to quarantine and usually won't have a problem. But some will quarantime and some will occasionally have a problem.
