if it is Marek's then you will probably see other outbreaks from time to time. Generally here in the UK it seems to be less virulent than some of the strains in the USA, so I would be surprised if you suffer significant losses. Keeping flocks stable and happy is important. Chopping and changing flock members about, adding new birds, moving them etc all increases the risk because stress is usually a trigger.... adolescent cockerels running around chasing the girls, integrating chicks into an adult flock, surge of hormones at point of lay, change of hormones during moult. Obviously some things you can control, others you can't. You may lose young birds suddenly without showing any obvious symptoms or you may have an outbreak of coccidiosis or respiratory disease. These can all be indications that their immune system is compromised by the virus, but in my experience most stay strong and healthy.
