Mites: spray your birds directly with a permethrin solution -read directions for the dilution rate. I use a garden sprayer bc it sprays a fine mist. You can do it with one person, but we have one person that holds bird upside down, the other person sprays vent and between legs and under wings. We do it in the daytime bc we will begin with a full clean out and spray the entire coop, and put in new bedding, then the birds, and they get put into the closed and clean coop. This way only sprayed chickens in the clean coop and we know who remains to be sprayed. Repeat in 10 days to get newly hatched (not yet mature) mites. No egg withdrawal with permethrin-so eggs safe to eat!
Medicated feed for chicks: helps them to develop natural resistance to coccidosis. The medication keeps the illness from getting a hold on them, then they will develop a natural immunity over time. Many recommend introducing chicks to your dirt in the first two weeks to get their immune system going on developing resistance.
Mareks stays in the soil for awhile. We visited a chicken keeper once who walked all through her coop and run area then told us these were the Mareks survivors!!!

Luckily we were on a trip and threw away our shoes (old ones, thankfully), washed our clothes a couple times before going home.
Many healthy birds are sold at swaps etc, so she may not have brought any illness to your place.
If your birds free range, then keep them away from neighbors place.
If the run stays wet or quite moist, add several inches of more bedding, etc.
Good luck