Sudden infant death syndrome is marked by the sudden death of an infant that is not predicted by medical history and remains unexplained after a thorough forensic autopsy and detailed death scene investigation. As infants are at the highest risk for SIDS during sleep, it is sometimes referred to as cot death or crib death. Typically the infant is found dead after having been put to bed, and exhibits no signs of having suffered. SIDS was the third leading cause of death of infant mortality in the U.S. in 2011. According to the Centers for Disease Control and Prevention, SIDS deaths have been declining since 1988. The cause of SIDS is unknown, but some characteristics associated with the syndrome have been identified and appear to interact with other characteristics: A triple-risk model states that SIDS occurs when an infant with an underlying, biological vulnerability who is at a critical developmental age is exposed to an external trigger. SIDS prevention strategies include: putting the infant to sleep on his/her back, a firm mattress separate from but close to caregivers, no loose bedding, a relatively cool sleeping environment, using a pacifier, and avoidance of exposure to tobacco smoke. The "Safe to Sleep" campaign is considered a significant public health success, credited with leading to a measurable reduction in SIDS rates.