Sudden unexpected death in infancy (SUDI), also known as sudden unexplained death in infancy, occurs in babies younger than one year and is a leading cause of postnatal infant mortality. (Most cases of SUDI cannot be explained, and if death takes place while the infant is sleeping, the case may be deemed sudden infant death syndrome.) According to the Centers for Disease Control and Prevention, approximately 4,600 U.S. infants died of a sudden unexplained cause in 2004 (see http://www.cdc.gov/SIDS/SUID.htm).

To determine the possible role of infection in SUDI, British investigators conducted a systematic review of SUDI cases at a children's hospital in London from 1996 through 2005. They examined more than 2,000 fluid and tissue samples from 470 infants and found that 73% were positive for bacteria. Samples from infants who died of an unexplained cause contained significantly more isolates of Escherichia coli, Staphylococcus aureus, and Listeria monocytogenes than did samples from infants whose cause of death was known but had been deemed unrelated to infection. The authors note that although postmortem bacterial analysis alone cannot be used to determine the cause of death, its results may point toward a possible cause of death. "Microbial products rather than direct microbial invasion could in some way lead to SUDI," they write. They also suggest that these dangerous bacteria may underlie other problems, "such as overheating or impaired infant arousal responses."