SIDS is a syndrome whereby an seemingly healthy infant dies suddenly with for no medically apparent reason - typically during sleep.

90% of cases happen before six months of age, and it's the leading cause of post-neonatal infant mortality in many countries (e.g. the US and Europe).

The cause is currently unknown, though 'associations' have been found with environmental pollution - such as cigarette smoke. There are also possible genetic links, and some evidence that sleeping-position is a contributing factor.

A new (2023) research project from Boston Children’s Hospital, Massachusetts, US, found, in SIDS cases, abnormalities in the way that the neurotransmitter serotonin bound to 5-HT2A/C receptors in the lower brainstem. Ref.Journal of Neuropathology & Experimental Neurology, Volume 82, Issue 6, Pages 467–482, ^{[ paywalled ]}

It's now suggested that the syndrome could be the result of interactions of various abnormalities - possibly including brainstem defects. The exact details of which remain unclear.

Further info. US NIH