Did alleles for drug resistance occur in bacterial populations that afflict humans, prior to the widespread use of antibiotics in medicine and agriculture?

Yes, drug resistance existed before, but it increased after the  widespread use of antibiotics

Explanation:

Drug resistances in bacteria  usually occurs as a consequence of mutation. Individuals that carry the resistance mutation survive even after the treatment with antibiotics. So, these mutations are advantageous for bacteria and thus, become favourable by natural selection (natural selection is evolutionary mechanism which favours traits of an organism that help him survive and reproduce). Resistant bacteria that survived after the use of antibiotics had the favuoruable trait (resistance) by natural selection.

Bacteria can even transfer drug-resistance to other bacteria via the process of conjugation, transducton or transformation.

a "theory" a suggestion that is not 100 % accurate, or a figure. some people say its a theory because they of all the passed theory's about one subject. it depends sometimes theory might be accurate and sometimes they are.