BY LEE BOWMAN

Despite decades of research, no definite cause of SIDS is known. Researchers believe that Sudden Infant Death Syndrome probably has more than one cause, and that there may be different causes of SIDS from one baby to another.

Here are some of the possible causes of the syndrome that have been discussed and researched over the years:

Ammonia -- A theory that ammonia gas from urine in wet diapers builds up in the crib and affects respiratory tract and breathing.

Arrhythmia -- Loss of rhythm in the heart, perhaps brought on by changes in the autonomic nervous system during sleep.

Brain stem defect -- Infants have some abnormality or immaturity in the brain stem, which controls breathing and waking reflexes. This defect may not allow a baby to rouse when challenged by not getting enough oxygen or too much carbon dioxide.

Carbon dioxide rebreathing -- A baby rebreathing exhaled air over an extended period of time could result in oxygen deprivation.

Developmental disorders -- This theory suggests that some critical part of the brain or respiratory system does not develop properly from birth, leaving the baby susceptible to SIDS.

Environmental challenges or stress -- Some extra challenge to the body, from passive tobacco smoke to excess carbon dioxide or overheating or a viral or bacterial infection, triggers the breakdown in function of the central nervous system that controls heart beat and respiration.

Immune system flaw -- Some research has shown that the immune systems of babies with SIDS have a higher-than-normal number of white blood cells and proteins, which could interact with the brain to affect heart rate, breathing or arousal function during sleep.

Kidney disorder -- A disorder called nephron deficiency, which affects the organ's ability to excrete, has been linked to a number of infants who died of no known cause.

Metabolic disorder -- Theory that SIDS babies lack a certain enzyme that processes fatty acids. As a result, they experience a buildup of those acids that causes a fatal breakdown in breathing and heart function.

Toxic gas -- The notion that either chemicals in bedding or a common household fungus in bedding produces a toxic gas that shuts down the central nervous system.

Toxic infection -- Suggestion that SIDS may be linked to infection by a common bacterium, such as E. coli, which can produce a shock-inducing byproduct. The toxin was found in the blood of SIDS babies in a 2002 Italian study.

Vitamin C deficiency -- Australian researchers suggest that SIDS could be the result of infantile scurvy and possibly have an effect on proteins that guide electrical activity in the heart.