Introduction

It has been nearly 30 years since the publication of a seminal book that defined the state of knowledge related to the epidemiology of, and mechanisms underlying, sudden infant death syndrome (SIDS) (1). Despite decades of subsequent research, much of which is summarized in other chapters in this book, we must acknowledge that SIDS remains an enigma. Indeed, two longstanding definitions of SIDS (2, 3) are testament to our lack of understanding of why infants die of SIDS — that is, these deaths remain unexplained after thorough investigation. Although infrequent, SIDS remains the most common cause of infant death between 1 month and 1 year of age, and the deaths of 2,000 infants annually in the United States (US) alone are unimaginable tragedies for these 2,000 families. At the heart of the reason why we have such an incomplete understanding of SIDS is, fortunately, its rarity. In the US, the 2014 estimates suggest that SIDS is the cause of death for about 3.9 of every 10,000 infants born each year (4). Over the past few decades, our understanding of the external factors that contribute to why infants die of SIDS has come from numerous, worldwide, epidemiological studies. Associations gleaned from these studies have led to recommendations including strong discouragement for mothers not to smoke during pregnancy paired with specific guidance for safe sleeping practices. Subsequent to these recommendations, the rate of SIDS was reduced in many countries (5). However, the physiological mechanisms that underlie SIDS remain unknown.

By definition, SIDS deaths are unexpected. While there may be evidence of low-grade infection prior to the time of death (6) in general, there are no overt, chronic signs of the impending demise. These deaths do not seem to be “programmed”, in the sense that they are inevitable; rather, they appear to be due to suboptimal physiological regulatory responses to what may be rather common challenges faced by infants during the first year of life. Nonetheless, these deaths are not random. Some infants are more likely to experience the failure of adequate physiological responses to environmental challenges than others, hence the concept of the vulnerable infant. Infants born prematurely are at greater risk for SIDS (7), as are infants of mothers who smoked or drank during pregnancy (8, 9).