There is an infectious disease that is so common that one in three people on our planet carry it. That means if there are 30 children in your class, 10 of them have it. The good news is that only one in 10 of those infections will go on to become the active disease. The bad news is that the one classmate who does have the disease has only a 50/50 chance of surviving on his own. Around 2,000,000 people die from this disease every year. That’s about 10 for every letter in this book. It’s a killer.

Once upon a time it was called consumption. People thought it consumed the sufferer from the inside out. One of its main symptoms is weight loss and paleness. Sufferers may also have a fever and often feel tired. But by far the most common symptom is a persistent cough accompanied by blood.

Tuberculosis, the disease is called, also known as TB, and it can be treated. That’s why no one you know has got it. But people in the Developing World, in places like Africa, die from it all the time. They just can’t afford the drugs they need to survive.