At what mission does Arthur get TB?

The specific scene in which Arthur gets TB in RDR2 happens during the Money Lending and Other Sins III mission. During the quest, Leopold Strauss, the Van der Linde gang’s loan shark, tasks Arthur with collecting a debt from the Downes family.

Can Arthur avoid getting TB?

Can you prevent Arthur’s tuberculosis? The short answer is no. Both in RDR2 and in the nonfictional 1890s, the chances of Arthur Morgan overcoming such a severe case of TB would be slim to none.