Antibiotics are a group of medicines that are used to treat infections caused by germs (bacteria and certain parasites). They do not work against infections that are caused by viruses - for example, the common cold or flu. Antibiotics are normally only prescribed for more serious bacterial infections - for example, pneumonia. Most side-effects of antibiotics are not serious - for example, diarrhoea, or mild stomach upset such as feeling sick (nausea). Although some people develop a serious allergy to some antibiotics, this is rare.