Whatever may be lurking in the hatching medium, possibly transmitted from your hands or those little flies or whatever.
The fungus that is devestating frogs through much of the world probably originated in South Africa, the African Clawed Frog is immune to it and can carry it - and was heavily used for cheap pregnancy tests worldwide - and we didn't even know about the fungus until after frog populations started vanishing.
Just one example of the devestation that can result.
When a local population has evolved without a particular disease present, it has no reason to have developed natural defenses to the disease, and the result can be devestating.
That's why release of CB animals or re-release of wild animals should only be done by qualified scientists, and then only if the local population is not viable, and then only if the cause for decline of local population has been identified and corrected.
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I decided my old sig was too big.