Antibiotics are medicines that treat infections and diseases caused by bacteria. Antibiotics do not work against viruses. Penicillin is a well-known antibiotic. Antibiotics are part of a larger group of medicines called antimicrobials. Other types of antimicrobials are: antivirals — for treating infections caused by viruses antifungals — for treating infections caused by fungi antiparasitics — for treating infections with worms and parasites These medicines are all used to treat infections. Because antibiotics have been overused, many are no longer effective. Antibiotics should only be used if they are needed to treat a bacterial infection.