Birds can definitely be stressed durring and after a show, especially if caged in a hot or extra humid area...Extra care should be taken if these conditions exsist. Fresh water with an electrolite additive is helpful and a mister can really cool down the birds. As to disease/parasite exposure: Any bird that is shown should be quarentined before reintroduction to your flock or breeding pens. I find it a good time to deworm and treat for extrenal parasites, as well.
Classes range from show to show but always include pullets and hens/ cockerels and roosters. So hens show against hens, pullets against pullets, and so on...eventually a class winner may show against an older or younger bird.
To get started it is always best to go to a show, just to see how things work. When you are ready to show; Get a copy of the show rules and requirements well ahead of the show to in sure your birds meet any testing requirements and to help you place your birds into the proper classes....Left unsaid; Buy the best birds you can afford from a reputable breeder/showman....buying anything less than good birds will only insure you never win...Don't Buy Hatchery birds and expect to Win....PERIOD...Jmo