It is not airborne as far as I know, it's in the soil like @ChickCrazed
said, but I think they can give it to one another by steping in the poo and and spreading it around that way, so I would think that even tho you other chicks are in the same room but NOT THE SAME PEN, they should be fine, make sure you wash you hands and anything else that may come in contact with both groups every very good each time you or they touch anything in the sic pen including the pen itself.
I love your feather planter, that is so cool looking.
How log have you had her/him? Your strain would be different then her strain so the immunity would be different. Would treat the group he/she was with too if it were me. Thats 1 reason I quarantine EVERYTHING for 30 days.
Had him/her for about two months maybe a little longer he/she arrived maybe 1.5 or more months before the cx and they were here about a month then I took them outside and they got sick however I got a bunch of silkies and a polish from a fellow member inbetween that time none of them got sick one died but I thought that was overcrowding(silkies were in a tote then moved to a brooder pen but there were no signs of sickness and they never went outside that's why I ruled out cross contamination polish roo is in a seperate pen by himself.. Cx never came near him steve did but steve and cx never came withen 20 feet of each other... It's just odd I've had over 100 chicks play in the yard(all I hatched) non got sick is it possible the cx carried it from welp hatchery? As far as I can tell no chicken has come here with it unless the cx brought it here maybe wild birds? The flying kind? Mother in law did find a small dead bird near the brooder room