I am very grateful to C. Stern for welcoming me to his lab, for the biological supervision of my work, and critical reading of this manuscript, to K. Page for advice on the mathematical aspects, to O. Pourquié for sharing unpublished information, and to J. Stark and R. Callard for helpful comments. This work was funded by a PhD studentship awarded to CoMPLEX by AstraZeneca. We are grateful to AstraZeneca for their support.