(HealthDay News) -- Sudden infant death syndrome (SIDS) is a mysterious condition that causes some 2,500 infants in the United States each year to suddenly and inexplicably die in their sleep.

Infants who die of SIDS are typically otherwise healthy and exhibit no symptoms, says the Nemours Foundation. Although it is the leading cause of death in infants between the ages of 1 month and 1 year, doctors still don't know what causes SIDS.

SIDS often is diagnosed when no other symptoms exist and when no other cause of death can be found.

Research indicates that a combination of risk factors can raise an infant's risk of SIDS. While there are few known preventive measures, you can reduce an infant's risk by always putting the infant to sleep on his back -- never on his side or stomach.

Other risk factors may include:

For the mother:

• Using tobacco, alcohol or illicit drugs during pregnancy.
• Getting poor care during pregnancy.
• Being a teenaged mother.

For the infant:

• Being born prematurely or at low birth weight.
• Being exposed to secondhand smoke after birth.
• Overheating from excessive sleepwear or bedding.