But over the last 20 years, the decline has plateaued – in 2020, the rate was 93 infants per 100,000. And while in many countries improvements in healthcare, vaccines and improved drugs have helped to crack, and largely eradicate, a number of the other conditions that once killed thousands of small children – measles, mumps, polio, and whooping cough, to name a few – sudden overnight deaths largely remain a mystery.