Like Non-Cancer terminal illness that can affect young (age 10-13) girls?, I have a major girl that needs to have a likely-terminal, repeatedly hospitalized, disease, but has a healthy sister. And it needs to still be deadly even if it appears in a world with effective and quick genetic engineering tech.
I was going to advance all medical tech, but that's a bit too difficult, so I'm sticking to just the genetic sciences.
It being uncaught by any screening, and only treatable by new genetic engineering tech in the final years (story start time) of her previously expected lifespan, so a "miracle cure" plot would be fine.
The leading answers in the question that inspired mine, were...
- Duchenne Muscular Dystrophy
- Congenital heart disease
- Cystic fibrosis
- Juvenile Tay–Sachs disease
- Rett Complex
I suppose only "Congenital Heart Disease" would be the hardest to cure even with Current Med / Gene Tech +5 Years. But are there more?