Bacteria are single-celled microorganisms that have been on the earth since the beginning of life. They are complex, sophisticated, metabolizing, self-reproducing, living organisms, and are very different from viruses.
Many bacteria exist in the biosphere and are essential in maintaining local and global ecosystems. Often, they exist in a harmonious balance with the host, providing the host with necessary chemical reactions to digest food and synthesize necessary nutrients. Then again, there are the bacteria that are the agents of disease.
Fortunately, many bacteria are sensitive to antibiotics, a class of substances that are capable of inhibiting and/or killing other microorganisms. Some bacteria produce their own antibiotics as a form of defense to protect themselves from other bacteria!
Unfortunately, primarily through abuse and overreliance, bacteria are beginning to show unusual resistance to traditional antibiotics. Thus, it is important when using antibiotics to insure that the antibiotic is appropriate for the indicated infectious agent and to complete the course of therapy.