EDIT: TLDR circa Decoy321 'Cureing' is preserving food often by using smoke. 'Cureing' is also healing medical conditions.
Smoking CAUSES medical conditions, and it is the joke that curing heals when smoking hurts.
For anyone unfamiliar, curing is a term for using salt and other ingredients to preserve/season food. The salt removes moisture, decreasing the ability for spoilage-inducing microbes to spread. Salmon is often cured to make it last longer.
I think that's enough explaining of the joke, you get the rest.