This book investigates the complex biological relationship between humans and microbes, specifically addressing the distinction between harmless microorganisms and pathogenic threats. John H. L. Playfair, an expert in the field, utilizes his extensive background in immunology to demystify how the human immune system functions as a defensive mechanism. He presents a framework that balances scientific rigor with accessible explanations, aiming to clarify the reality of infection versus the sensationalism often found in public discourse.

Experts and readers alike note that this text successfully bridges the gap between academic immunology and public understanding. The prose is frequently cited for its clarity and ability to distill complex biological interactions into an approachable format for the general reader.