Mythiclabs, get the vaccine, I use Twincitiespoultry. Vaccinate those day olds and keep them separate-quarantine for a few weeks. Your Cornish sound like it's right out of a book. You can see it at 4 weeks. I read it takes only 2 weeks to multiply, most get it 6 weeks and up. But there's no rule. It will be safe to eat them. You may lose all of them. Or a few. It's up to the disease.