SIDS is a medical name. The letters stand for Sudden Infant Death Syndrome. It is when a baby who seemed well dies during sleep, and doctors cannot find a reason — not an accident, not a known illness. The baby stops breathing and does not start again. It is talked about with very young babies, usually in the first year. American English also used crib death; British English used cot death. Those older names are the same idea. Nobody catches SIDS from someone else; it is not a germ. Advice about safe sleep, like putting a baby on its back, has made it less common in many countries, but the cause is still not fully understood. You say it as four letters: S-I-D-S. This is a heavy, careful word. Use it for this exact medical idea, not as a soft way to talk about every death of a baby.