Antibiotics are chemical substances that kill or inhibit the growth of bacteria. The success of antibiotics against disease-causing bacteria is one of modern medicine's great achievements. However, because bacteria adapt quickly to new environmental conditions, many bacteria harmful to humans have developed ways to circumvent the effects of antibiotics, and many infectious diseases are now much more difficult to treat than they were just a few decades ago. Critically ill patients are more likely to require the aid of antibiotics to fight infections, so are more likely to be harmed by the spread of antibiotic-resistant bacteria.

Bacteria contain genetic material called plasmids, which can carry the genes enabling antibiotic resistance. Bacteria share these plasmids among one another via a direct, mechanical transfer between cells, and antibiotic-resistant plasmids can thus spread easily throughout a bacterial population to create a strain of resistant bacteria. Less commonly, a natural chromosomal mutation may confer antibiotic resistance on a bacterium, which can then reproduce and become dominant via natural selection, likely when that colony is exposed to antibiotics. In the absence of human involvement, however, bacteria rarely develop resistance to antibiotics.

On January 1, 2006, the European Union banned the feeding of all antibiotics to livestock for non-therapeutic purposes. This sweeping policy followed a 1998 ban on the non-therapeutic use of four medically-important antibiotics on animals. In the United States, by contrast, animals raised on industrial-scale factory farms are still routinely administered low levels of antibiotics in their feed-not as a cure for ongoing maladies, but primarily as a growth-enhancing agent to produce more meat and also as a prophylactic measure to compensate for overcrowded and unsanitary conditions. Currently, several antibiotics that are used in human medical treatment, such as tetracycline, penicillin and erythromycin, are also administered non-therapeutically to healthy livestock and poultry. This long-term non-therapeutic use of antibiotics in the United States creates the ideal conditions for the development of antibiotic-resistant bacteria, as the drugs kill only the susceptible bacteria, leaving the resistant strains to reproduce and flourish. The newly-resistant bacteria can then spread from farm animals to other animals, including humans.