Antibiotics by definition are anything that opposes life. Antibiotics are substances used to inhibit the development of certain bacteria in the human body. Certain antibiotics, like penicillin, the first antibiotic discovered in 1929 by Fleming, are historically natural, but are today, for the most part, synthetic antibiotics. Antibiotics act through different mechanisms depending on their nature and aim to block the proliferation of bacteria by hindering the next stage of their development. Antibiotics are prescribed only in cases of bacterial infection, and can be combined for more severe infections. Antibiotics must be prescribed prudently, as bacteria are developing ways to resist antibiotics and lower their efficiency.