unexpected death of an apparently healthy infant from unexplained causes. SIDS is of worldwide incidence, and within industrialized countries it is the most common cause of death of infants between two weeks and one year old. In 95 percent of SIDS cases, infants are two to four months old.

Sudden infant death syndrome occurs almost always during sleep at night. Its cause remains unknown. From the time of its identification, researchers have posited a number of causes—from a theory (popular in the 1960s and since discredited) that SIDS was caused by parental neglect to suggestions that SIDS was triggered by childhood vaccinations, blood disorders, and apnea (a disorder in which breathing is arrested during sleep)—but none has been borne out by further research. A higher incidence of SIDS is seen among premature and low-birth-weight infants, as well as among those born to teenagers, women who smoke heavily, and those who have received poor prenatal care. In the late 1980s researchers began to examine the brain development of infants, theorizing that some abnormality in the process of learning a response to respiratory distress would explain the syndrome.

Because studies have shown a higher incidence of SIDS among infants who sleep on their stomachs, physicians now recommend that infants be positioned to sleep on their back or side.