One medical historian has traced this idea back to Louis Pasteur, noting that the most “efficient” parasite, in Pasteur’s view,9 was one that “lives in harmony with its host,” and therefore latent infections should be considered “the ideal form of parasitism.” Hans Zinsser voiced the same notion in Rats, Lice and History, observing that a long period of association between one species of parasite and one species of host tends to lead, by evolutionary adaptation, to “a more perfect mutual tolerance10 between invader and invaded.” Macfarlane Burnet agreed: In