When checking for coccidia in droppings the actual count is important. A bird that has built some immunity can be carrying some and not be sick. When the numbers get so large that they overwhelm the immune system, then the bird gets sick. Anytime a new bird is brought in there is a risk that it may bring in a previously unexposed strain to your existing flock, and that your flock/ground may contain a strain that the new bird has never been exposed to, and there is no immunity or resistance. Since her flock needed a sulfa drug to treat it, then that particular strain is probably going to require that med to treat in the future as it appears it may be resistant to amprolium. I would have that on hand since that strain may have already been transferred to your ground in droppings from those birds.
When I am raising chicks (I have dealt with coccidiosis outbreaks, and know it's in my soil) I start the very first week in the brooder giving them a large plant saucer of dirt from my yard (not from the chicken run) to dig, scratch, peck and dustbathe in. They make a mess, but love it. I gradually add in a bit of soil from the chicken run in small amounts. This gives their immune systems a chance to build resistance naturally, just like if they had a broody mama and were on the ground right away. It doesn't guarantee that you will not have an outbreak, I always have meds on hand in case, but it has greatly reduced my incidence. I've not had an outbreak in almost 4 years (knock on wood) with multiple batches of brooder raised chicks. Bringing in adult birds or older birds will be different, and you will just have to watch closely for any signs of illness, and treat if necessary.
