In the seventeenth century, smallpox reigned as the world's worst killer. Luck, more than anything else, decided who would live and who would die. That is, until Lady Mary Wortley Montagu, an English aristocrat, moved to Constantinople and noticed the Turkish practice of "ingrafting" or inoculation, which, she wrote, made "the small- pox...entirely harmless." Convinced by what she witnessed, she allowed her six-year-old son to be ingrafted, and the treatment was a complete success-the young Montagu enjoyed lifelong immunity from smallpox. Lady Montagu's discovery would, however, remain a quiet one; it would be almost 150 years before inoculation (in the more modern form of vaccination) would become widely accepted while the medical community struggled to understand the way our bodies defend themselves against disease. William Clark's At War Within takes us on a fascinating tour through the immune system, examining the history of its discovery, the ways in which it protects us, and how it may bring its full force to bear at the wrong time or in the wrong place. Scientists have only gradually come to realize that this elegant defence system not only has the potential to help, as in the case of smallpox, but also the potential to do profound harm in health problems ranging from allergies to AIDS, and from organ transplants to cancer. Dr. Clark discusses the myriad of medical problems involving the immune system, and he systematically explains each one. For example, in both tuberculosis and AIDS, the underlying pathogens take up residence within the immune system itself, something Clark compares to having a prowler take up residence in your house, crawling around through the walls and ceilings while waiting to do you in. He discusses organ transplants, showing how the immune system can work far too well, and touching on the heated ethical debate over the use of both primate and human organs. He explores the mind's powerful ability to influence the performance of the immune system; and the speculation that women, because they have developed more powerful immune systems in connection with childbearing, are more prone than men to contract certain diseases such as lupus. In a fascinating chapter on AIDS, arguably the most deadly epidemic seen on Earth since the smallpox, Clark explains how the disease originated and the ways in which it operates. And, in each section, we learn about the most recent medical breakthroughs. At first glance, it may appear that our immune system faces daunting odds; it must learn to successfully fend off, not thousands, but millions of different types of microbes. Fortunately, according to Clark, it would be almost impossible to imagine a more elegant strategy for our protection than the one chosen by our immune system, and his At War Within provides a thorough and engaging explanation of this most complex and delicately balanced mechanism.