Sorry, I was at the grocery store
Most parrots that are raised as pets are hand raised. Ethical breeders work our butts off to keep the babies healthy, but the process of hand feeding (as well as parent raising) is one that involves a lot of warm wet food; VERY conducive to bacteria and yeast. It's just the way it is, even when parent birds are allowed to raise their own babies. It's actually that way in humans, too. Ever heard a mom say her human baby has "thrush"? That's a yeast infection in the mouth.
Basically babies don't yet have super strong immune systems and the feeding process can fuel infections even with the most careful of feeders, and even more in a lax setting. The stress of weaning (most breeders and nearly all pet stores use the cruel practice of force weaning) and going to a new home further weakens the immune system, opening wide the possibility for illness. That's why I always urge people to take new birds, especially babies, to the vet immediately for a checkup with gram stains. Without them, an infection will almost always be missed and they are quite vulnerable.
So don't worry, this infection isn't some harbinger of doom, it's a routine illness. Follow the advice of your avian vet to the letter. Ask him or her about avian probiotics, and be glad you took your baby in while the infection was still minimal
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