Microbes have killed more people than any war, and also made most of the food in your kitchen possible. This Open University series, compiled here from its original seven parts, moves between those two facts: the Black Death and cholera as case studies in what a single-celled organism can do to a population, and fermentation and baking as case studies in what the same organisms do for us every day. It treats bacteria, viruses, and fungi as one connected subject rather than splitting disease from biotechnology, tracing why some microbes cause epidemics while close relatives are cultured deliberately in yogurt, cheese, and bread. The throughline is neither fear nor reassurance but function: what a microbe actually does inside a body or a vat, and why that determines whether we fight it or farm it. It closes on the practical question the Open University builds courses around, how the same biology drives both antibiotic treatment and antibiotic resistance.