Complex sentence: Genes for resistance to antibiotics, like the antibiotics themselves, are ancient; however, the increasing rates of antibiotic-resistant bacterial infections seen in clinical practice stems from antibiotic use both within human and veterinary medicine.
Split into simpler sentences: Genes for resistance to antibiotics, like the antibiotics themselves, are ancient. However, the increasing rates of antibiotic-resistant bacterial infections seen in clinical practice stems from antibiotic use both within human medicine and veterinary medicine.