While in my opinion it is best to be left as a mystery, ilness would be the most likely cause like @MurineBombs64 suggested already.

I think she died of a sickness

Abigail most likely died of tuberculosis, although it's never explicitly stated. She has a persistent cough in between missions during the Beecher's Hope stage of the game.

Suicide. I already mentioned my idea in an earlier post

@MadMikeRyan

Doesn't necessarily have to be Tuberculosis. It could be Lung Cancer.

@Equivalent-Ambition True enough, but TB's more likely. I'm not sure what the initial survival time for lung cancer is, but if Abigail was already symptomatic I doubt very highly she would have lasted a full 3 years.

I doubt she’d last three years with either TB or lung cancer. Her having a cough in 1911 really isn’t much to go on and it’s very far-fetched to attach it to her death. It’s no secret that grief can weaken the immune system, so I imagine she died of some illness. Hard to say what that could’ve been; all we know is that Jack didn’t get it, so it can’t have been hugely contagious. She could’ve also had cancer, as it can be caused by changes in hormones and is not contagious at all, so that’s also plausible.

@Wagwan piffting23 I don't usually like to speculate since as you've said it causes people to make far-fetched attachments, but I've done a bit of reading up and TB definitely fits: It would seem that the most common initial survival time for TB before treatment was discovered was around 6 months, though it tended to vary case to case. However there have also been numerous reported cases of individuals surviving for several years untreated, and even going into remission for prolonged periods of time. If Abigail was in the early stages of the disease in 1911, it wouldn't be a stretch to think she could have lasted 3 years. Most forms of cancer however would have taken her sooner. As for Jack never contracting the disease, it's not unheard of for people living in the same home as an infected person to avoid getting sick, and lets not forget the time setting and place being when TB was not uncommon, and therefore people were more suspicious of the symptoms. It's very possible Abigail's illness was caught early and she and Jack were careful about it. But I suppose we will never know.

@MadMikeRyan Yeah, especially since TB was incredibly common until much later, and even now it still is one of the biggest causes of human mortality in the world.

Most likely tbc , it was a mass murderer back in those days . I still don’t understand why no one else in the gang got tuberculosis due to beeing in Arthurs presence and lacking any proper hygiene or decent healthcare . It was very infectious . But no one got sick for some strange reasons . In the real world they wouldn’t even need the pinkertons , just let nature do its work and the Whole gang would be gone in no time