Diseases and parasites can spread by them sharing the same dirt or scratching dirt into adjacent runs, sharing food or water containers, vectors like mosquitoes, grasshoppers, grubs, snails, and many others, or by blowing in the wind. You can carry diseases or parasites on your clothing (especially shoes), by using the same buckets to carry food and water to each group, or dipping different buckets into the same feed container.
Some flocks develop what are called flock immunities. They have a disease or parasite that they can give to other chickens but have developed an immunity to it themselves. Coccidiosis is a good example but there are others. No matter how long you quarantine them they don't show symptoms. This can be your current flock as easily as it can be your new chickens. Your current chickens may make your new ones sick.
Quarantine is a powerful tool when used correctly. The problem is that most of us can't do a real good quarantine, we just don't have the facilities. Knowing how diseases and parasites are spread set up the best quarantine you can. It probably won't be perfect but it will be better than nothing.
If these new birds have been in a closed flock where they have not been exposed to other birds for over a month, if the person taking care of them would recognize a parasite or disease, and if they are honest enough to tell you if they know anything the birds have basically been in quarantine, just not at the new place or under your eyes. To me, quarantine is not that important for these birds but if they come from an auction or chicken swap where they have been exposed to new birds the risk is much higher.
Most people quarantine for 30 days. Some diseases can take longer than that to show up, sometimes months, but 30 days will catch most of them. To me it's a case of doing the best you can. Perfection usually doesn't exist in real life.
