SIDS is a kind of God-of-the-gaps for infant death. We can't figure it out, therefore SIDS. But we now have a better explanation for crib death
Sudden infant death is as much of a "syndrome" as accidentally suffocating to death as an adult.
Most cases of Sudden Infant Death (SID) are when the child often sleeps in an unsafe sleep environment : cramped areas, places with low-quality air (smoker's homes for instance) usually the baby dies while sleeping on it's stomach (the prime position for suffocation). I believe that many cases of SIDS is actually due to sleep apnea. Where the individual breathes less during sleep, and occasionally stops breathing entirely.
SIDS is just a name we give to something we can't Identify, like UFO. UFO's aren't actual things, they are just aircraft that the person calling it a UFO can't figure out what kind it is and/or who is in it. SIDS is not the cause of death, it's usually suffocation. But the doctor can't figure that out so he says SIDS because he can't just tell you he himself doesn't know at the time.