SIDs, while still too common, has dropped 70% in the U.S. since the "back-to-sleep" effort and the general awareness of the situation. Children under 6 months don't have a reflex to carbon dioxide poisoning, and their respiratory systems aren't fully developed, so it is easier for them to suffocate on their own breath pooling in too-soft mattresses or cribs with polyester blankets or bumpers. Risk factors for SIDs death include: winter, cigarette smoke exposure, respiratory illness, not breast-feeding, and not sleeping in the same room as an adult.
Keeping an infant too hot is a risk factor. In winter people may use extra bedding, or thicker sleeping bags, or extra clothing, or reduced ventilation, or increased heating.
