Haven't made a TB post in a while so I thought I'd touch on Arthur's illness again, specifically how it was possible for the gang to have not gotten the illness too (since its a frequent question as to how that was possible).
First off, what is tuberculosis and what are the types of tuberculosis? There are many, many different types so I'll colour code the relevant two.
Tuberculosis (TB) is caused by a bacterium called mycobacterium tuberculosis (Mtb). The main cases we see are latent tuberculosis infection and active pulmonary tuberculosis.
Latent tuberculosis (LTBI) is a tuberculosis infection, the bacterium is dormant/inactive in the body.
Active tuberculosis (ATB) is a tuberculosis disease, the bacterium is active in the body.
The main difference between the two is whether the bacteria are dormant or actively causing disease. People with latent tuberculosis infection have no symptoms (are asymptomatic) and cannot spread the bacteria to others, whereas people with active tuberculosis disease usually have symptoms and may be able to spread the disease - especially if the infection is in the lungs. Both are handled differently, but both are treated with antibiotics at various stages.
Infection = the bacterium, virus, etc, is present in the body.
Infectious = the ability to pass a sickness, disease, etc, to others.
Now, back to Arthur's illness.
Arthur is a frequent discussion in our field funnily enough. Mostly because he is a highly unusual TB patient, but we love him for that.
Arthur's tuberculosis likely remained latent or at least minimally active for the first half of the game after his encounter with Mr. Downes, eventually progressing into active pulmonary tuberculosis. The thing about latent tuberculosis is the constant risk of it becoming active tuberculosis if/when the body is put under an immense amount of stress and strain. (Looking at you Dutch)
It would explain how, for the most part, Arthur had little to no symptoms even days/weeks after his initial (and brutal) exposure. He still went on missions, still mingled with the gang, still talked to people, and there were no signs of illness spreading. Though after the events of late chapter three/chapter four onwards, Arthur's illness definitely developed and became active tuberculosis.
But how come it didn't spread? Well at that point Arthur knew he was ill in some form and was already subconsciously distancing himself because of it, and also distancing himself because of the emotional events going on/the tension between Arthur and the others.
Realistically, should the illness have spread later on? Yes, but not in the way that seems obvious. By chapter six, some members of the gang would likely have been exposed to Arthur's tuberculosis. Though it is important to remember that exposure doesn't always mean infection, and infection doesn't always mean disease.
Many people exposed to tuberculosis never develop active symptoms for a very long time, if any at all.
The gang were very fortunate that the illness didn't spread the way it could have, I think it was mostly down to pure luck and convenience with the story.
The game was very careful about emphasizing Arthur's deterioration whilst trying not to punish the player for continuing with the story and risk impairing the gameplay, and it was done wonderfully! It's so rare to have such a raw and aching representation for a very real illness, and to see how it affected a prominent main character such as Arthur himself.
Apart from the coughing and visible decline, I think the illness forced Arthur to slow down for once and watch everything he had built his identity around be stripped away piece by piece.
But that's a post for another day, hopefully this has been sorta interesting!