Explore the microscopic world of viruses in this captivating Christmas Lecture by epidemiologist Jonathan Van Tam. Delve into the nature of these invisible invaders, their impact on our bodies, and the revolutionary advancements in testing that are reshaping modern medicine. Join immunologist Katie Ewer and virologist Ravi Gupta as they unravel the complexities of viruses, from their basic structure to their role in human genetics. Discover the intricate workings of our immune system, including the functions of mucus, antibodies, B cells, and killer T cells. Learn about groundbreaking developments like the AstraZeneca vaccine and the mechanics behind PCR and lateral flow tests. This comprehensive 59-minute lecture, originally broadcast on BBC4, offers a fascinating journey through the invisible realm of viruses and their profound influence on human health and medical science.