I could not get this to work in the Rail Nomads Camp after trying 25+ times. Plus, based on the outcome of the quests in the camp, the woman there may choose to leave town.
Instead, I got this in Rodia, which you can go to after reaching California. In the back of the Casino there are several prostitutes that take bullets as payment, and the first one triggered the STD. I'm not sure if they stay there if you run the Leather Jerks out of town, but there will be another chance in Hollywood as well.