In general, "western" novels tend to be light on actual history, so I haven't read many. I made an exception for Lone Star Rising: The Texas Rangers Trilogy (2003), by Elmer Kelton, after reading a very interesting straight history work called Empire of the Summer Moon : Quanah Parker and the Rise and Fall of the Comanches, by S. C. Gwynne (2011).
For a good account of west Texas history in the middle 1800s, get Gwynne's book instead. Readers might question the characterization of the Comanche as "the most powerful Indian tribe in American history", but Gwynne makes a strong case.
P.S. A few more thoughts about why this novel didn't mention smallpox or cholera. "Western" novels have some archetypical characteristics. One of those is anthropocentrism: the view that "man is the measure of all things". That viewpoint can't stand up to the fact that disease had far more to do with the near-extinction of native Americans than any deliberate actions by humans.
A dualistic good-evil morality is another staple of western novels. The "good" people struggle to meet and overcome challenges, while the "evil" people act badly. Smallpox and cholera, however, don't care who's good and bad. The universe doesn't care about humans. That point of view doesn't work in a western novel.