Indefinite article and verb-noun agreement

singular, and you don't need an "a"...

Many sources, including the Chicago Manual of Style, say that when you have an "or" alternative subject, the verb should agree with the part of the subject nearer to the verb. In other words, "diseases" not "one," is closest. Therefore, the correct verb is "promote". The indefinite article, "a", is not required.

Doubtful, I agree, for what it's worth (not much lately)

In the Reg of the Bistro, Phil, we Ibises encourage the Central Ion of ...