Although antibiotics truly revolutionized medicine in the 20th century, there is evidence of antibiotic use since ancient times. Today, these antimicrobial substances are the most important type of antibacterial agent for fighting bacterial infections by either killing or inhibiting the reproduction of bacteria. When the bacterial cause (pathogenic microorganism) of an infection is identified, a definitive antibiotic therapy is used. When the cause is unknown, a broad-spectrum antibiotic therapy may be prescribed based on the patient’s symptoms. Antibiotics are also used as a preventative measure for topical application to surgical wounds, etc.