The fundamental observation that led to the development of immunology as a scientific discipline was that an individual might become resistant for life to a certain disease after having contracted it only once. The term immunity, derived from the Latin immunis (exempt), was adopted to designate this naturally acquired protection against diseases such as measles or smallpox.
The emergence of immunology as a discipline was closely tied to the development of microbiology. The work of Pasteur, Koch, Metchnikoff, and many other pioneers of the golden age of microbiology resulted in the rapid identification of new infectious agents. This was closely followed by the discovery that infectious diseases could be prevented by exposure to killed or attenuated organisms or to compounds extracted from the infectious agents. The impact of immunization against infectious diseases such as tetanus, measles, mumps, poliomyelitis, and smallpox, to name just a few examples, can be grasped when we reflect on the fact that these diseases, which were at one time significant causes of mortal-ity and morbidity, are now either extinct or very rarely seen. Indeed, it is fair to state that the impact of vaccination and sanitation on the welfare and life expectancy of humans has had no parallel in any other developments of medical science.
In the second part of this century immunology started to transcend its early bound-aries and become a more general biomedical discipline. Today, the study of immunologi-cal defense mechanisms is still an important area of research, but immunologists are in-volved in a much wider array of problems, such as self-nonself discrimination, control of cell and tissue differentiation, transplantation, cancer immunotherapy, etc. The focus of in-terest has shifted toward the basic understanding of how the immune system works in the hope that this insight will allow novel approaches to its manipulation.