For more than half a century, vaccines have quietly transformed human life. They have prevented suffering on a scale that is difficult to imagine, protecting millions of children from diseases that once claimed countless lives. Many of the most dramatic public health victories of the modern era have been built on vaccination. Diseases that once filled hospitals, overwhelmed families, and devastated communities became rare or disappeared entirely in many parts of the world. Yet vaccination now stands at a fascinating moment in history.

Every second of every day, your heart performs an extraordinary balancing act. Roughly the size of your fist, this remarkable organ contracts about 100,000 times each day, pumping thousands of litres of blood through an intricate network of vessels. It does so with astonishing reliability, rarely taking a break from the moment it first begins beating before birth until the end of life. Most of us think about the heart in simple terms. We measure its rate, check our blood pressure, or perhaps hear about blocked arteries and heart attacks. Doctors often assess how much blood the heart pumps with each beat or calculate the familiar “ejection fraction,” which estimates how well the heart empties during contraction. These measurements are valuable, but they tell only part of the story.