In my family, it was Cystic Fibrosis--among other things.
Before I was born, a brother and a sister died as infants. They weren't diagnosed with CF. My sister, Loa Elizabeth died of peritonitis when she was only days old.
But my little brother had CF. He was very bright and very, very fragile. He loved to go to school, but it was difficult for him to sit up-right at a desk. His favorite game was pretending to fish with a stick and a piece of string.
By the time I became a high school teacher, science had advanced enough that I had a student with CF. He was very bright and, despite the skater swagger, very fragile.
Science certainly progressed during the years between my little brother's death and my student's life. My student was eligible for experimental trials that hadn't even been imagined when my brother died.
You can learn more about cystic fibrosis here.
Addendum: The world lost Eli.