You need to quarantine new birds and keep them separate from your "established" birds. This helps ensure that the new bird isn't sick and, in case the new bird is sick, prevents that bird from infecting your other bird(s).
This means when it is time for cage maintenance, doing your established bird's cage first, THEN the new one, and washing your hands after handling the new bird before handling the established bird.
This quarantine period should be at least a month. Some potential diseases will take awhile to show themselves, so it's important to be patient. This DOES also give the new bird time to get used to you and its new environment before meeting the rest of the flock, though, as well.