Early in this century, patients with cancer would often seek medical attention only in the final stages of their disease, after their tumors had become massive. Surgeons would attempt to remove these tumors to alleviate their patients' pain. But since sterile operative techniques were in their infancy and the discovery of antibiotics almost half a century away, such surgery often caused massive and frequently fatal infections.
In a few instances, however, tumor remnants of infected patients would disappear, leaving them healthy and doctors puzzled. Today, after decades of research into how the immune system works, scientists have learned that the seemingly miraculous cures resulted from the infections themselves, which set in motion complex immune reactions.

Understanding the relationship between cancer and the immune system-whose purpose is to find and destroy any abnormal cells in an organism among a mass of normal ones-has resulted partly from so-called "experiments of nature." A few diseases involving immune deficiencies are associated with higher rates of cancer, for example. A person infected with HIV, for instance, is 100 times more likely to develop a malignancy than an uninfected person. Also, organ-transplant recipients treated for a long time with drugs that suppress the immune system, thereby preventing rejection of the new organs, are many times more likely to develop cancer than the rest of the population. In fact, an elegant demonstration of the immune system's role in protecting against cancer is the fact that tumors occasionally regress when doctors remove immunosuppressing drugs.