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.

Select from the choices above to read the entire article.