Most antibiotics used by modern dairy farms are also important human medicines. As antibiotic resistance can spread through the environment, development of resistance at a dairy farm has the potential to reduce efficacy of these medicines in humans. To address this concern the World Health Organization (WHO) has established a list of Critically Important Antimicrobials for Human Medicine based on two criteria: 1) the antimicrobial is the sole, or limited, therapy, to treat serious bacterial infections in people, and 2) the antimicrobial is used to treat infections in people caused by bacteria from non-human sources or by bacteria that may acquire resistance from non-human sources.