While the human medical use of antibiotics is the main source of antibiotic resistant infections in humans,[42][43][44] it is known that humans can acquire antibiotic resistance genes from a variety of animal sources, including farm animals, pets and wildlife.[45][46][47][48] Three potential mechanisms by which agricultural antibiotic use could lead to human disease have been identified as: 1 - direct infection with resistant bacteria from an animal source; 2 - breaches in the species barrier followed by sustained transmission in humans of resistant strains arising in livestock; 3 - transfer of resistance genes from agriculture into human pathogens.[49] While there is evidence of transmission of resistance from animals to humans in all three cases, either the scale is limited or causality is hard to establish.