The last thing I want to do here is scare people. Knowing the
source of your chicks or hatching eggs and then isolating them
for several weeks will prevent most infections and/or contaminations.
Most diseases will appear in under 30 days so you will know it.
I've hatched eggs from at least a dozen BYC members and none
were infected. Brooding the chicks for several weeks was also
a form of isolation so if they turned up sick then I would have
culled the batch.
What I did was:
1-Buy a young silkie pair from a kid and his dad who had way too
many birds packed into their property and were active with 4H and
other shows. The birds were all well taken care of. He just had a
lot. AHappyChick also got birds from him that introduced Mareks
into her flock. She has gone through heck with her birds and now
vaccinates them all herself, even her hatchlings.
2-The bird noticeably sounded bad. I ignored this. Smart huh?
3-Brought them home and put them in my isolation brooder which
is in the same barn as all my chickens and almost immediately
mixed them up. Again, smart huh?
We can't avoid our flocks being infected with Cocci or Mareks from
a wild bird but we can avoid what happened to me.
Again:
1-Know the source
2-Isolate and monitor
3-Practice biosecurity when exposed to other flocks