Bibliographical noteFunding Information:
The important contributions of many investigators could not be specifically cited due to space considerations, but many of these can be found within the bibliography. We thank Tim Behrens, Bill Brown, Eric Hendrickson, Stéfan Jónsson and Michael Neuberger for thoughtful comments on this manuscript. R.S.H. is supported in part by a Searle Scholarship, a University of Minnesota McKnight Land Grant Assistant Professorship, and a grant from the NIH NIAID (AI064046).