Language is central to the development of science. Ideas are communicated through language, but they are also defined by language, and nowhere more so than in the case of immunity. The double meaning of immunity—resistance from infection or disease, but also exemption from a rule or penalty—inscribes a legal element into our scientific understanding of the body and its systems. The parallel between physical and social bodies, the ways in which they are defined and policed, was not difficult to see.