Maybe cystic fibrosis? From what I understand, the disease basically makes it so that you always have a cold and eventually causes your lungs to collapse because you can’t recover. Most people diagnosed have multiple lung transplants and don’t live past 25-30. Your body has issues with producing the cells that make mucus, sweat, and your digestive system… so then these things become thick and sticky and simulate cold-like symptoms. There’s no known treatment for this one, though.
Another disease that could kill you (except this one will cause death within a week or 2) is meningitis. That bacteria destroys your nervous system I’m pretty sure. If it’s not caught quick enough, you can lose limbs due to dead tissue and well, you know, die. The treatment for this is antibiotics, but you’ve really gotta catch it quick bc this bacteria is nasty.