**Core Concept**
Sudden Infant Death Syndrome (SIDS) is a sudden and unexplained death of an infant under one year of age, with no clear cause or explanation. It is a leading cause of death in infants between one month and one year of age.

**Why the Correct Answer is Right**
SIDS is often associated with a combination of factors, including sleep environment, maternal smoking, exposure to tobacco smoke, and prone sleeping position. The exact cause of SIDS is still not fully understood, but research suggests that it may be related to abnormalities in the autonomic nervous system, which regulates breathing and heart rate. One of the key risk factors for SIDS is the prone sleeping position, which increases the risk of rebreathing of carbon dioxide and hypoxia.

**Why Each Wrong Option is Incorrect**
**Option A:** SIDS is often associated with a combination of factors, including sleep environment, maternal smoking, exposure to tobacco smoke, and prone sleeping position. This statement is true, so it cannot be the correct answer.
**Option B:** SIDS is caused by a viral infection. While some studies have suggested a possible link between SIDS and viral infections, the exact cause of SIDS is still not fully understood, and this statement is not accurate.
**Option C:** SIDS is more common in infants who are breastfed. Research has actually shown that breastfeeding is associated with a lower risk of SIDS, so this statement is incorrect.
**Option D:** SIDS is the same as cot death. SIDS and cot death are often used interchangeably, but cot death is a more general term that refers to any sudden and unexpected death of an infant, whereas SIDS specifically refers to a sudden and unexplained death of an infant under one year of age.

**Clinical Pearl / High-Yield Fact**
To reduce the risk of SIDS, the American Academy of Pediatrics recommends that infants be placed on their backs to sleep, with a firm mattress and a tight-fitting sheet, and that parents avoid exposing their infants to tobacco smoke and other environmental hazards.

**Correct Answer: B. SIDS is caused by a viral infection.**