This is the eBook version of the printed book. This Element is an excerpt from Antibiotic Resistance: Understanding and Responding to an Emerging Crisis (9780131387737) by Karl Drlica and David S. Perlin. Available in print and digital formats. What everyone needs to know about antibiotics: what they are, how they work, what they can do, and what they can’t do. Antibiotics are selective poisons. They are relatively small molecules (about 20-100 times the size of water molecules) that interfere with normal life processes of microbes and viruses. Human cells differ enough from pathogens for antibiotics to act selectively. For example, our cells lack walls, whereas bacterial cells have them. Consequently, penicillin, which blocks cell wall synthesis, is specific to bacteria....