Quarantine is placing the new bird in a separate area, with different heat/air flow. Also, clothes should be changed, hands washed. No sharing!

Why Quarantine?

This is probably one of the most mis-understood necessities of owning
birds. When one is quarantining a bird or birds it is to protect the old
flock as well as the new. Each group of birds live in their own unique
environment, and have built up immunities to the germs (good and bad) that
they are exposed to daily. Regardless of where a new bird is purchased
from, or how impeccable the husbandry or the reputation of the seller,
quarantine should be regarded as a very necessary practice.

The new bird is crated and taken from its eco-system and placed in a
totally new environment with a multitude of germs (good and bad) that it
has never been exposed to. During this move to the new location some stress
will be experienced. This stress can be very minor or it can be a major
upset, depending on the nature of the bird, the difference in environment
and how the bird reacts to it. During this time of stress, the birds
immune system may become suppressed, and the bird may not be in as good a
physical shape as when it left it's home. If the bird is not quarantined,
it will be bombarded by millions of new germs and the immune system will
need to kick in and respond to all these new germs (good and bad). With a
compromised immune system the bird will not be able to surmount a good
response and may indeed fall victim to a germ that normally would not be
pathogenic (disease causing) in this bird in a different situation.

The new bird now becomes ill and starts shedding vast amounts of this now
(new to him) pathogenic germ, and also starts shedding germs in vast
amounts that the bird brought with him from his old environment. We now
have millions of pathogens in the environment that the resident birds are
being exposed to. Some are new germs, and some are old that they had
immunities to, but the shear volume is more than they can handle. Now we
have old and new birds getting sick, and of course one believes this
disease came with the newest arrival.

Obviously, if any of the birds involved had an existing pathogenic disease,
the consequences would be much worse. Why Quarantine?

Had this arrival been quarantined properly, his stress level would not have
been so great and his immune response would have been able to build up to
the smaller amounts of germs it was exposed to. After a gradual time of
small exposures, the immune system can build immunities at a much more
normal pace, and not become compromised. This gradual transition into a new
environment proves beneficial, and necessary to all the birds involved.
Germs don't read one way signs.

By author Jean Patterson