I start with dirt from outside the chicken run. Then I start adding a small amount from the chicken run, keeping in mind the idea of gradual exposure. I have not had an outbreak in about 4 years since doing this with all brooder raised chicks. They get exposed this way the same as if they were raised by a broody hen. I do keep Corid on hand whenever I have chicks. Having dealt with it in the past I want to be prepared to treat quickly if there are any signs, but I've not had to do it for some time. But if I do have one get sick, I'm ready to treat right away.