Anti-infective agents are drugs designed to selectively target and kill invading microorganisms without harming the host's cells. Paul Ehrlich was the first scientist to develop a synthetic chemical with this property in the 1920s. Anti-infectives work by interfering with microbial cell walls, protein synthesis, DNA synthesis, or cell membranes. Their goal is to reduce microbial populations to levels the immune system can handle. Microbes can develop resistance by modifying drug targets or transport mechanisms. Careful dosing and limiting inappropriate use can help prevent resistance.