Sam finds out because of Jess.
She’s in love with her Genetics class, and with her Immunology class, and her Advanced Proteins class - and basically every one of her biology classes that explores how the human body works. So when she learns about genetic screening, she immediately wants to try it.
Sam’s less keen, but when Jess turns her big blue eyes on him and begs for it as a birthday present, his defences crumble.
He doesn’t expect to find out about the FFI gene.
Jess lasts about ten minutes before the first tear rolls down her cheek.
It takes him all night to talk Jess down, to point out that the average age of onset is fifty, that there are ways to improve quality of life, that biological research is constantly advancing, and there may even be a cure by the time he’s affected.
Jess is working with a professor studying prions and genetics by the end of the week.
Sam calls his father and Dean once a week for nearly three months, leaving voicemails about what he’s found and begging them to get tested. If they’re lucky (and he hates himself for thinking it) he got the gene from his mother, and John is safe, while Dean only has a 50% chance of having the dominant gene.
Jess bites down on the instinct to point out that if either Mary or John had two dominant alleles of the gene, then Dean would automatically share the same fate as Sam. There’s no way to know for sure unless Dean gets tested, and Sam needs all the hope he can get.
In 2006, Jess is dead, John is dead, Dean doesn’t have the gene, and a paper comes out about a man with FFI who extended his lifespan by a year. Sam has it bookmarked ten seconds after finding it, and spends months hijacking university networks to access all the related research he can. (He finds Jess’ only published paper from undergrad and stares at it for an hour, remembering how hard Jess worked on it, how many nights she spent on the couch with him trawling through research articles while he wrote his own essays, how proud she was when it came out in PNAS, three days before he got his LSAT score.)
Six years later, Sam can’t sleep and Lucifer’s doing his level best to remind him of the Cage. The doctors pump him full of sedatives that don’t work and only make the hallucinations worse, and Sam remembers with dread barbiturates only exacerbate the clinical symptoms -
Twenty-eight years old, and he’s going to die.
It’s more time than he ever thought he’d have.
It’s not enough time at all.
Cas takes Lucifer away, but the insomnia stays, and Dean dies before Sam can figure out how to tell him.
He’s sitting in the Impala trying to figure out what to do, if he should try and get Dean back, if Cas is really gone, when he tallies up the past months and realises he’s already in the last stage of the disease.
There’s no more time.
Soon he won’t be able to move or speak, and when he finally loses the ability to breathe, he’ll die. This isn’t the Cage, where Lucifer can just bring him back (though maybe when this is all over and he returns to the Cage, Lucifer will.)
Sam gets in the Impala and drives.
He keeps driving until he hits a dog, and he can’t let him die, doesn’t want to let his last act be a mistake. So he takes him to a vet, demands the doctors take care of him, and stumbles out with a sad smile when a feisty redhead tries to make him take it home.
He’s already losing the ability to speak and walk.
There’s no more time.
He parks the Impala at the side of the road and walks until he reaches a cliff. It’s beautiful out here, wild and open and free. The sky is cloudless and blue as a robin’s egg. There’s a note in the Impala asking that she be taken to a good home, to someone who will appreciate her beauty and worth, instead of sitting in an impound lot somewhere, gathering dust.
Sam takes one last breath, and jumps.