Finished: Slocum's Snake Oil by Jake Logan (who really knows?), 2010,
Used paperback I picked up somewhere.
Slocum is in Eastern North Dakota and hard up for cash. When a drunken local starts blathering about a big money shipment Slocum decides to rob the stage. The robbery was a set-up by the Federal Marshall and Slocum is pursued across the prairie. Slocum heads straight into a buffalo herd, starts a stampede, survives the running cows and emerges from the dust to find a medicine wagon. The medicine show peddler helps Slocum out and Slocum agrees to help the "Doctor" out for a few days.
Slocum and the Doctor land in a town stricken with typhus (it might not have been typhus, I don't have the book handy). Slocum meets a woman delivering her dying husband to the town doctor. Widow left her son at the farm. Slocum rescues the boy from marauding bandits taking advantage of the deadly typhus outbreak. Slocum and widow have sex. "Doctor" leaves town
Slocum finds out the peddler's elixer actually works. Wow! Slocum travels after "Doctor" to save Woman's son and the town. Slocum still pursued by Federal Marshall. Chases ensue. Gunfights ensue. Sex ensues. "Doctor" says, "Really? It worked?" "Doctor" and Slocum travel to find Indian Medicine Man who shared the elixir recipe. Mean Indians Ensue. Slocum hunts for ingredients. More sex. More violence. Slocum brews the elixir himself and saves the town.