Here are two kinds of diseases that affected cattle in a cowboy's life.
Screw Worm Cattle Disease
Cowboys had to constantly check the herd for evidence of screw worm larvae laid in open wounds by blowflies. This insect plague caused the eventual death of massive amounts of cattle in the Old West.
This was a cattle disease caused by a Texas "tick" that the Longhorn cattle themselves were immune to, but it infected other cattle in Kansas and Missouri. In Missouri state laws were passed to prevent Texas cattle from infecting the local herds, and Kansas vigilantes formed as early as 1855, which led to some very dangerous armed conflicts. In 1866, quarantine laws were enacted to keep Texas cattle out of eastern Kansas, which resulted in the emergence of the western cattle towns such as Dodge City becoming cattle drive towns for a period of time.