Entire populations have been wiped out by diseases like bubonic plague, typhus, and cholera, because no one knew how they were transmitted. Knowing that bacteria or viruses are the agents of transmission is only one part of the solution, though—witness the famous influenza epidemic in 1918 that killed 21 million people; we must also know how to cure the disease or prevent it. This program shows how bacteria multiply, distinguishes between those which are beneficial and those which are inimical to the normal functioning of the body’s various systems, and shows how invading bacteria trigger the body’s defense mechanisms so that white blood corpuscles or antibodies are produced to destroy the invading organism. The program also explains the differences between bacteria and viruses, and the ways in which immunization serves to provide antibodies to protect against future attack by bacteria or viruses. (20 minutes)