With Mareks it honestly depends on who you ask lol. Most research today says that mareks is so wide spread that if you own poultry for a long period of time your eventually going to have to deal with it. It typically (not always) but typically does not really affect your birds after sexual maturity, they can still get it but its typically not fatal, its an age thing so your adult birds may even have it and you dont know. The vaccine after 1 day is also probably up for debate on who you ask. It says 1 day but I know people that will give it to adult birds just to be safe. I personally will give it to young juvenile birds. You also have those that argue Mareks vaccination at its core because your technically giving the bird a form of Mareks and even more will argue with you that they can pass on Mareks from the vaccine itself and about 1-5% of birds will die from the vaccine. Its a very hotly debated topic depending on who you ask. I have had birds die from who knows what over the years and reality is with chickens if you have enough and long enough you will have some die and you will have no clue why. Obviously its very different if large numbers of your birds die which luckily I have never had to deal with. The way we handle it is relying on the aging process and its worked great for us. All new birds and young birds stay separate from the rest of our flock until they reach about 6 months old and laying age then they go in with the older birds. That 6 months of quarantine I guess you can call it has worked for us really well. Plus if I have a bird that starts acting odd or lethargic or really odd in any way I immediately remove and kill it. I know alot of people love their chickens and try to doctor them and medicate or whatever, I dont do that. I live by the old standard of only the strongest survive because I only want strong birds breeding so I dont really doctor my birds unless its an injury I can help with but anything else we just cull immediately. Diseases are scary thats for sure and vaccines are a hot debate, just like in people I guess some think they are great some refuse them, I suppose its a risk either way. Not commonly know the vaccine does not even stop the bird from getting mareks, there is no vaccine or cure for it, all the vaccine does is hopefully stop if from being fatal if they get it. Luckily Mareks is not transmitted to eggs so even if mom and dad have it and you get chicks they wont have it unless they get it from your flock or wherever they go. Our hatchery room is is a completely different building to once again try and quarantine day old chicks from older birds so our day old chicks never come within about 50 yards of our older birds from hatching to shipped out. Reality is you can be as careful as you want and then some person could come over to visit you and bring it on their shoes or pants right into your barn. We try and have a lock down barn in that nobody but the 3 of us (wife and son) are ever in the barns. That works out for us because we also own and breed livestock guard dogs so nobody can get inside our fencing and be in the barns anyway. I hope that answered your questions. I figured I would write alot since other can read the reply as well in case they had the same question. Oh and yes NPIP number will be on packages once we start shipping again in 2015.