Medicine for the disease
Unfortunately, there is no cure for this disease. No matter how the player tries to protect the health of the hero, this fate is the idea of the screenwriters. That's what they think, the story of redemption told and shown in RDR 2 must end. In the story, after discovering the diagnosis, Arthur completely changes his outlook. And it can't help but affect the gameplay itself.
Perhaps the only thing that can be done to somehow affect the health parameters of the main character is to make him amulets from legendary animals.
The finals
In addition to the realization that Arthur is waiting for a certain end, comes the understanding that the gameplay is complicated. The player will find that the main character will die faster, his running speed will slow down, which will not facilitate constant skirmishes and battles with enemies. Each time it will be harder for him to climb on the horse.
It is very sad that there is no way to rid him of this disease. It was possible to assume that after all that had happened, he would simply mount a faithful steed and disappear over the horizon. But no.