‘Fighting disease’ looks at how the immune system and vaccination work. It considers the scientific studies of Louis Pasteur, late in the 19th century, who brilliantly expanded on the work of Robert Koch and Friedrich Henle, to formally expound the germ theory of human disease. But how did the body defend itself against these micro invaders? The phagocytic theory of immune defence resulted from the work of Élie Metchnikoff, Paul Ehrlich, and Emil von Behring. Immunoglobulin molecules provide the key to how the body creates the variety of molecules needed to protect against the different invaders experienced over a lifetime, and to how vaccination against a disease protects against future infection.