Antibiotics are a class of drug that are specifically designed to kill bacteria that cause infections and various illnesses. Some antibiotics are broad spectrum and can target many types of bacteria, whereas some are narrow spectrum and target only certain strains of bacteria.
Antibiotics are approved in the US for use in food-producing animals to promote growth, increase appetite, prevent infection, as well as to actually treat animals who are ill. This practice has been strongly associated with antibiotic resistant infections in humans. Many antibiotics are broad spectrum, therefore taking antiobitics can harm beneficial bacteria in the body as well, causing vaginal infections (yeast infections), upset stomach and diarrhea.
Antibiotics cannot treat illnesses caused by a virus, such as the common cold or the flu.