I've first heard of the term "Sudden Death Syndrome" (SDS) in relation to large Central American cichlid hybrids. Sometimes when you mix and match different species, the same unfavorable gene combination safeguards that keep species true will prevent successful hybridizing. Those "bad genes" or "deadly genes" will sometimes allow gestation and birth to happen; but will trigger death on an apparent healthy individual at some young stage in it's life. To me, it makes more sense the use of SDS in those cases, such as with unstable "Green Apple" Pacman hybrids. Maybe it's just a case of semantics; but will try to explain my point.
Internal organs on young organisms work usually fine, unless they don't. Today's incredible advances in human medicine show that many "sudden death" of the past are attributable to Heart or neurological issues. In those cases the organisms (humans) did not die a sudden death; they died because of a life threatening health issue.
Pet medicine has made great advances, but when it comes to frogs, we are still struggling along. Our frogs can die of acute Heart or neurological issues, same as we can. And when they sadly do; we can't say they were healthy. Unless in a research labs, no frogs get echocardiograms to check their Hearts or circulatory system and they do not get MRI's to check out their brain functions either. And even when conducting a necropsy; a veterinary could miss some stroke or cardiac symptoms.
When an apparently healthy frog suddenly dies, it's not SDS, it was unhealthy and we just did not knew it

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