I would ask the source if they have Mareks present in their flock.  That is one disease that you don't want to have present.   It is a form of herpes, and stays on your  grounds like forever.

 Well,
 not forever
, but is hard to  get rid of once present.
I can direct you to some threads I follow  on the subject of Mareks, or you can easily find them on BYC also.
Bottom line.... Its not the Apocalypse,,, but much better to avoid if possible. 
I did a kwik search and this may calm your mind..
Copy & paste
Therefore, although immunization of chickens with live Eimeria oocysts is effective and has been the basis of live oral coccidiosis vaccines for almost 70 years, chickens must be vaccinated with oocysts from each Eimeria species in order to be fully protected.
Lil Extra Roo

may have gotten a bug that was outside of the range of those that vaccine protects against.

Similar to us getting a flu shot. It only protects against  the most predicted strains (I think 5 or 6 strains) that the  Medical World assumes will be active in the coming flu season.
I'm pretty confident Lil Roo  would have gotten  the mareks vaccine if he was shipped to household that ordered vaccinated chicks. JMO.