New chickens need to be quarantined away from your other chickens for at least 30 days. Make a place for them to live alone away from your current flock for an extended time. They should not occupy the same breathing space as your current flock. I keep new birds in a dog crate in my green house. The dog crates are easy to clean and dissenfect between birds.
Each flock of chickens has their own germs that make them immune to certain things in their environment. So and So's flock back east will be immune to different microbiology in their immediate environment that is different than my flock in California. That is to be expected. The same is true of any flock no matter where they are. Immunities differ from flock to flock and area to area.
During the quarantine:
1. Observe for any signs of illness or disease.
2. Practice good hygiene and wash your hands a lot!
3. Give new birds a supplement in their water to boost their immune system. Give them good probiotics - such as a dish of yogurt.
5. You might consider giving them a little extra protein as they will be stressed being in a new place and might drop some feathers or a little weight.
Always take care of your old flocks first then wash your hands, spray any feces off your boots so as not to track it into the new birds area, if possible spray disinfectant on boots then go see to the needs of your new birds. Wash your hands again and make sure your boots are clean.
Moving and rehoming chickens is stressful on them. It is during this time of stress that any illness or disease they might be carrying will manifest itself. Disease can take up to a month to show up in a seemingly healthy chicken. Many Many of these common diseases make a chicken a carrier for life.
Some sellers are not aware their birds are ill, and some illness do not present themselves untill the birds are under stress. During quarantine, the main things to look for are lice, mites, breathing problems, discharges from eyes or nostrils, fungus type patches on the combs and wattles, raised scales on the legs, indicating scaly leg mites. ets. They should not share the same airspace as your existing flock because some diseases are airborne, as in coughing and sneezing, etc.
I tell people who buy my birds to quarantine. To my knowledge, they have never had anything communicable at all, but what happens if they have just contracted something and haven't shown symptoms yet for me to know? Quarantining is just a good practice.