In the western world, antibiotic resistance can arise through different methods: clinical use and agricultural use.
In agriculture, antibiotics have (and are) used as prophylactic growth enhancers. By suppressing the growth of an animal’s microbiome, it is possible to allow more nutrients to be taken up by the animal, increasing the efficiency of the meat growth process. However, due to the constant low-level use of antibiotics for this purpose, it is easy for bacteria to develop resistance. Via agricultural staff and those handling the animals, it is possible for these bacteria to transfer to human populations. This has been observed with MRSA in the Netherlands. Many commercial poultry producers in the US mandate their suppliers use specific feeds and antibiotics to obtain higher efficiencies while growing the animals.
Antibiotics are also used widely in human medicine. In some jurisdictions, it is possible to purchase antibiotics without talking to a clinician, and this can lead to antibiotics being used against viral infections or bacteria that will not be killed by them. Some clinicians may make incorrect decisions about which antibiotic to use, and for how long, when prescribing to a patient. This also can increase the likelihood of resistance developing.