I was watching for Marek's too.

This article is quite prescient for me because it's incubation season and I just lost a stunning number of chicks that I ordered pre-hatched to what was obviously a virus (identical care conditions to other breeds I was simultaneously hatching rather than ordering pre-hatched). All but 4 or 24 dropped dead within three days.

However, I deliberately keep a heterogeneous flock (2-3 breeds from rotating sources) so no other birds were even slightly ill.