Immune systems are the same no matter what, whether it be pampered kids, pedigree dogs or racing pigeons. Kids that are never allowed out to play and get dirty never get the chance to develop their immune systems, farmers kids are among the healthiest. Pedigree dogs that are kept in kennels and only taken to shows are the same, mongrels are among the healthiest so why do you expect pigeons to be any different.
Like us when we go on holiday, locals can drink the local water and eat all and local food but it can upset visitors big time because their systems can't cope with the difference in bacteria etc. If you go to countries that have things like polio, typhoid, malaria etc you get injections before you go to build up your resistance
Stress is a major killer, if you break a bone you can die from the stress of the injury but not the actual break. If your immune system is week then stress can bring on underlying problems that are already there, vaccinations are taken or injected using a part of the infection so your immune systems are allowed to fight it and build up antibodies to fight it and increase your immune system. Surely when you vaccinate pigeons for PMV you are introducing this into their bodies so they have resistance to fight it off, then mothers should pass this on to their young, but we then inject some more into them, maybe this is the reason the birds are becoming weaker and PMV is on the increase :-/
