No bird hobbyist in their right mind would pass on a sick bird to another hobbyist, nor to a trader who may hold us liable. It would be a foolish thing to do, because sooner or later there will come a time when we are confronted with the consequences. Moreover, it is a sure way to discourage beginners.
A diseased bird should be isolated from the rest, preferably in a special cage with extra heating. In many cases the bird is not capable of producing sufficient energy itself to maintain its normal body temperature of 42 ºC.
Make an account of the symptoms exhibited by the bird, record them and then consult a bird specialist or avian veterinarian. Together discuss the required course of action: whether the bird itself should be examined by the vet for diagnosis, or whether it might be sufficient to provide a fresh excrement sample for analysis. Preferably collect such a sample in the afternoon, because that is when the concentration of parasites is highest.