sudden infant death syndrome (SIDS) or crib death, sudden, unexpected, and unexplained death of an apparently healthy infant under one year of age (usually between two weeks and eight months old). SIDS accounts for 10% of infant deaths and is the second highest cause of death (after accidents) in infancy. The risk is higher in males, in low-birth-weight infants, in lower socioeconomic levels, during cold months, and for babies who sleep face down.

Causal theories suggest that the infant may have immature or hypersensitive lungs, may have a defect in brain-stem control of breathing, or may be rebreathing carbon dioxide. Recent studies have shown persistent high levels of an infant form of hemoglobin in babies with known risk factors for the condition.

SIDS victims are thought to have brief episodes of apnea (breathing stoppage) before the fatal one. An alarm system that detects breathing abnormalities is sometimes used with infants suspected of being prone to SIDS. The American Academy of Pediatrics has recommended that babies be laid to sleep on their backs or sides.