I have what is called a "closed aviary".  This means that all my
birds were pruchased at one time, tested for ALL avian  diseases
and then  placed in the aviary after  testing negative for  such
diseases.  I  went the extra  step and paid  the extra money for
DNA probing to make sure the birds tested clean of all diseases.

Some  diseases go through  what is known  as a "shedding"  phase
where the  disease is active and  able to be picked up on tests.
At other times the disease may be in a type of "remission" where
it cannot be picked up by normal testing, but it can be seen  on
a DNA probe.  

More specifically the DNA probe is looking for what is called  a
"tider".  A  tider is the left over  remains of a disease in the
DNA.  People have  the exact same thing.  Once we  get sick from
a certain  strain of a disease  and our body gets rid  of it  on
its own  or with the help of  antibiotics,  a tider is left over
and the human body realizes what it is and we can never get  the
exact same strain of that specific disease or illness again.

If a  bird comes  back positive for  a DNA tider,  it means that
the bird had  the disease or was exposed to  it and in that case
it will NEVER come into my aviary!

Needless to say ALL of my birds are clean!