Please visit the following website for a good write-up on the two:

http://www.internationalparrotletsociety.org/pbfd.html.

At risk of oversimplifying things, young birds that are just beginning to fly or soon after seem to die suddenly after just sudddnely losing their appetite or vomit and dropping tail and wing feathers or abnormal feathers are more likely to be infected by Polyoma. If they recover and their feathers turn normal after their next moult, then it was probably Polyoma. If the feathers get worse after each moult, it is likely PBFD. However about 20 -30% recover spontaneously. Usually older birds withstand both diseases better. Somehow the two diseases seem to affect the birds immune system - think bird-AIDS! Some are carriers - shed the virus intermitently but don't show any other signs.

Note that both diseases are very infectiously - expect the dry feather particles to be infective and stressed birds shed lots of virus!! For this reasons, avoid bringing young birds to bird-shops - cramped wild birds are perfect sources of infection.

Rupert