Because bacteria are so adaptable, the more they are exposed to antibiotics, the faster they are able to change and develop resistance to those antibiotics. There are some bacteria that have become so strong and so good at evading antibiotics that there are very few choices left to treat the infections they cause. The more bacteria are exposed to a particular drug, the faster they can develop a resistance to it. Antibiotics are commonly given to animals in their feeds to promote weight gain. This practice also contributes to antibiotic resistance, although the impact on human health is controversial.
Antibiotic resistance is spread by the bacteria themselves.
Resistant strains of bacteria appear in countries around the world much more quickly now because of people's increased travel and mobility. Antibiotic resistance occurs because bacteria are living things and constantly evolve and adapt to conditions in their environment. The actual process by which bacteria evolve and change may sound a bit like science fiction but the fact that a new generation of bacteria is produced every 20 minutes makes it possible for antibiotic resistance to develop very quickly. Pharmaceutical companies are continually developing new types of antibiotics that are more effective than those that have limited effect on infections because of antibiotic resistance. However, it takes many years to develop and test a new antibiotic to make sure that it is both effective and safe for humans, and this is an extremely expensive process.