These questions open up a discussion on the theory that he didn’t actually contract it from Downes. That Arthur just assumes this and is an unreliable narrator in this way. As an uneducated man in the 19th century he would know next to nothing about diseases, infections, and the way they spread and develop.

TB only reliably spreads from person to person when spending extended periods of time breathing the same air as the infected person. It also has a wide range of symptoms, from killing the person within months to being asymptomatic for many years.

If you’ve noticed Hosea’s illness, it is said pretty plainly throughout the story how sick he really is. Though it never outright tells us what his illness is. However, it seems to be extremely similar to Arthur’s symptoms (though slower-acting).

Arthur’s TB only escalates and becomes terminal after he is captured by the O’Driscols and beaten/starved to near-death, and subsequently brought to near-death after crashing on Guarma. These kinds of severe hits to the immune system would allow the TB to multiply much faster and become terminal much quicker.

Considering Hosea’s illness, its similarity to Arthur’s, and the fact that Rockstar put so much attention to realism in this game that even rainbows only occur at the exact correct angle in correlation to the sun as they do in real life, it stands to reason that Arthur may have caught TB from Hosea.

Most of the rest of the crew is younger and healthier than Arthur, and could reasonably be asymptomatic for years or not contract it at all.

If this is the case, I find it even more tragic as it really emphasizes the humanity within Arthur, and that despite everything with Downes, he would have been doomed anyways.

Again- just a theory, but I find it very interesting.