If you buy birds intended as layers they will have probably already been vaccinated
it is possible to vaccinate before the chicks hatch now machines are able to ingect hundreds of eggs a day
whether you vaccinate or not is entirely up to you, you need to consider several factors such as
do you visit shows or auctions, how valuable or rare your birds are, is a particular disease prevelent in your area and does someone else keep fowls within a mile or so? do wild birds have frequent access etc
and you need to bear in mind especially with Mareks the vaccinated birds will carry and shed the vaccine virus for life (they can also pick up another strain and carry that to) and will shed it to other birds to, just like an unvaccinated bird will pick up the virus and shed it to others but vaccinated birds are unlikelly to develope clinical disease.
personally I will always do it now as i can't stand the heart break when a really good bird starts to reach maturity and then develops Marek's
I have tried nursing them through it several times. even using antivirals but they have always died, Sometimes it takes 6 weeks but it is relentless and always wins
Breeding for resistance is all very well in theory but if there is no natural resistane to a disease in the gene pool you are working with then that particular disease can come along and wipe most of them out.
lets imagine you have 100 birds of a particular breed. Disease comes along and kills 95 of them you can be fairly sure the remaining 5 were able to fight the disease and so, in theory, be able to pass this ability to their offspring.this may be acceptable in utility breeds where the only nessecity is for them to lay large numbers of eggs or put on weight quickly but if they are a rare breed or of show stock do the remaining birds conform to the standard of the breed in colour shape size etc ? becouse all these factors need to be taken into account as well.
if you are lucky they might, but what a price to pay?
personally I have neither the time, depth of pocket or space to go down that road
The opposite is easy to acheive as is what happens with an outbreak of foot and mouth,
All cattle in the area of an outbreak are killed every time the disease appears
so for generations the animals never come into contact with foot and mouth therefore resistance is never challenged. so if in the future the disease does strike it will hit them like a 'bolt from the blue'
This is what happened when the europeans introduced small pox, syphilis and influenza to the native Americans and Hawian peoples. Neither had any natural resistance and the populations were devastated.
Europeans had some resistance to these diseases having been exposed to the virus since time began -exposure to 'mild' strains giving helping to give immunity to virulent strains
also bear in mind if it wasn't for vaccination we would still be at the mercy of such killers as smallpox,diptheria and polio amongst others
OK i'm stepping off the soap box now