Some bacteria infect both animals and humans, and there is also horizontal gene transfer between bacteria, so if someone is infected with two (or more!) differe

Some bacteria infect both animals and humans, and there is also horizontal gene transfer between bacteria, so if someone is infected with two (or more!) different bacteria, (e.g. in a hospital environment - see https://pubmed.ncbi.nlm.nih.gov/30248271/ https://pubmed.ncbi.nlm.nih.gov/30248271/), that can result in the relevant resistance genes to be transferred to another disease, creating a strain of it that is also resistant to that antibiotic.