That's true. It's always a gamble.The last group of rescues that i evaluated for a humane society had Mycoplasma synoviae, and were all euthanized as unadoptable.
You can have some testing done on live birds, and it might save your entire flock if you do.
Better yet, don't get these birds, and plan on heathy chicks from a good hatchery in spring.
Being paranoid about biosecurity is a very good thing!
Mary