284  A Doctor’s Dictionary The classic text on the dilemmas presented by yellow fever in the colonial development of central America can be found in François Delaporte’s The history of yellow fever: an essay on the birth of tropical medicine, in its able translation by Arthur Goldhammer (1991).

Parasites [Northern Review, 1996] Years later after writing this essay, which reflects on my experience of studying parasitology at the London School of Hygiene and Tropical Medicine, I discovered amongst the brilliant aphorisms and observations of the Viennese physician and writer Arthur Schnitzler (volume 5 of his Collected Works) the following entry: ‘Notions such as “parasitizing”, “corrupting”, “extorting” are generally understood solely in a material sense. But aren’t there parasites attached to our being, ones much worse than those which help themselves to earthly goods? Aren’t there forms of corruption more subtle and even more underhand than those which exploit money and values? And aren’t the worst forms of blackmail those which attack our feelings or attempt to?’ It might even be thought, following on from Schnitzler, who was writing well before the Second World War, that our culture has become parasitic, feeding on its own past, and not always in an edifying way. As for the hybrid nature of human beings, it would seem that at least part of our DNA has been picked up from seemingly utterly alien species: a study in Genome Biology suggests that the ABO antigen system which codes for the basic blood groups is bacterial in origin, the gene associated with fat mass derives from marine algae and that the group of genes which synthesize hyaluronic acid, an intracellular binding substance, was captured from fungi. Twenty years ago, only a few specialists were interested in the gut: now the microbiome, the ecology of symbiotic microorganisms that inhabits our inner body space, is one of the trendiest topics around, and not just because antibiotic resistance has become medicine’s ‘global warming’ equivalent. The unsung symbionts of our gut provide us with a not insignificant part of our calorific intake, manufacture vitamins and co-factors and protect us against pathogens.