Why do we respond differently to infections or vaccines? Why are some people allergic to pollen? These are still unanswered questions in biological and medical science. The Milieu Intérieur Laboratory of Excellence coordinated at the Institut Pasteur by CNRS research director, Dr. Lluis Quintana-Murci, has recently described immune variation on a large scale within the French population. To achieve this, the consortium studied an expansive collection of biological specimens from 1,000 French volunteers aged 20 to 69. This article provides an account of their work.