Antibiotics, also known as antimicrobial drugs, are medicines that can kill or inhibit the growth of bacteria to cure infections in people, animals and sometimes plants. Antibiotics are medicines for bacterial infections (such as pneumococcal pneumonia or staphylococcal bloodstream infections); antimicrobial drugs that are effective against viruses are usually called antiviral drugs (such as those for influenza, HIV and herpes). Not all antibiotics are active against all bacteria. There are more than 15 different classes of antibiotics that differ in their chemical structure and their action against bacteria. An antibiotic may be effective against only one or multiple types of bacteria.