Poor infection control in hospitals and municipal areas allows resistant strains to spread unchecked between patients and communities. Global Health and Economic Implications

Antibiotic resistance is fundamentally an outcome of evolution. Any population of bacteria naturally includes variants with unusual traits, including the ability to withstand an antibiotic's attack. When a person takes antibiotics, the drug kills vulnerable bacteria, leaving behind—or "selecting" in biological terms—those that can resist it. These resistant bacteria then multiply rapidly, becoming the predominant microorganism. As Dr. Joe Cranston explains, "Whenever antibiotics are used, there is selective pressure for resistance to occur. More and more organisms develop resistance to more and more drugs."