Antibiotics are chemicals that help your body to fight diseases caused by bacteria and fungi. They are manufactured naturally by some microbes in order to protect themselves, but are now made synthetically. Different antibiotics work in different ways: some stop bacteria reproducing by preventing the formation of cell walls, while others prevent them from absorbing nutrients. Antibiotics have no effect on viruses.
Broad spectrum antibiotics work against many bacteria and those that work against specific microorganisms are called selective antibiotics. Overuse of antibiotics in human medicine and in animal feed has led some bacteria to mutate into forms that are resistant, such as strains of tuberculosis and so call superbugs.