Sudden infant death syndrome
In colder places where infants need help staying warm, the American Academy of Pediatrics suggests using a "sleep sack."  This is like a sleeping bag for a baby, but with holes for the baby's arms and head.  The bag is closed around the baby with a zipper.  This helps keep the baby warm without covering their head.  Sleep sacks are especially helpful for babies with low birth weight, who have more trouble keeping themselves warm.