Antibiotics or antibacterials are a type of antimicrobial used specifically against bacteria and are often used in medical treatment of bacterial infections. They may either kill or inhibit the growth of bacteria. Several antibiotic agents are also effective against a number of fungi, protozoans and some are toxic to humans and animals even when given in therapeutic dosage. Antibiotics are not effective against viruses such as the common cold or influenza and may be harmful when taken inappropriately.