For centuries, infections caused by bacteria were a major source of disease and death from illnesses such as pneumonia and tuberculosis. The discovery of antibiotics has proven critical in greatly reducing infectious diseases, and protecting public health relies heavily on the use of antibiotics. But repeated exposure to the drugs enables resistant strains of bacteria to evolve.
Some bacteria are naturally resistant, so they survive treatment and multiply.
When antibiotics are given again the resistant bacteria survive, and as their proportion of the bacterial population increases over time, the drugs become less effective. The more antibiotics we use, the more likely it is that bacteria will become resistant. With thousands of animals crammed into the unhygienic, crowded quarters of a typical factory operation, antibiotics are dispensed constantly through the animals' feed. Scientists do not understand how or why the drugs promote growth. Evidence is increasing that the emergence of antibiotic resistance, caused by overuse of antibiotics, threatens public health.