This electronic version has been made freely available under a Creative Commons (CC-BY-NC-ND) licence, thanks to the support of The Wellcome Trust. A copy of the licence can be viewed at https://creativecommons.org/licenses/by-nc-nd/2.0/uk/.
Monographs, or book chapters, which are outputs of Wellcome Trust funding have been made freely available as part of the Wellcome Trust's open access policy