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Germs are bad for you. They are also bad for medical history. Over the centuries doctors have struggled to win authority over the patient. In the late 19th century the germ theory of disease was a godsend. Doctors were able to point to germs - even pictures of them - as the causal agents of infectious diseases. Even more, killing the term meant curing the disease, and doctors did just that. The whole apparatus of ancient causality, the doctor's elaborate investigation of the patient's idiosyncrasies and environment went out of the window and the germ came in. Having identified the villain of the piece (the germ) the hero (the patient) was ignored. A new ontology of disease was at hand.

Germs are bad for medical history precisely because of the new authority they lend to the doctor. The doctor who understands syphilis seems to be the expert who can best understand its past. But such a doctor does not understand the new ontology: his "syphilis" is defined in the laboratory by microscopes and staining techniques; his measure of what happened in the past is a pathogen that did not exist in the perceptual and intellectual frameworks of the past. Not only that, but modern doctors say that syphilis often confusingly "mimics" other diseases and most old doctors gave clinical accounts of "syphilis" so brief that they could apply to plague. For these two reasons we cannot medically see syphilis in the past.

The authority of the doctor is also baneful when borrowed by others. It is because this authority derives from comparatively recent control of some infectious diseases that it carries with it a progressivist view of history. The medical historian knows that when Johannes Fabricius says the "dormant medical mind" was so shocked by syphilis that it initiated the change from magic to medical science, he knows it is because Fabricius has been intellectually infected by a historian in whose lifetime control of syphilis was achieved.

A related authority is that cultural bully of our times, psychiatry. The psychohistory that results from it has some anthropological similarities to progressivist history and both look like useful tools or baneful influences from another discipline, depending on who you are. it is not unknown in French psychohistory to analyse one's own dreams (about a mediaeval surgeon) in order to add to a documentary analysis (of the surgeon and surgery). Fabricius is content with an account of Shakespeare's mid-life crisis and his subconscious imagery.

These are two tools that a historian of literature does not need. On his own ground, Fabricius tells a good story. It is impressively researched and amasses surprising detail. Its verbal and pictorial images are numerous and striking. It adds up to a good picture of the perception of the disease by contemporaries. They did not confuse syphilis and gonorrhoea, because these are our categories, not theirs. Nor did they fail in an attempt to separate them, for they were not engaged in an enterprise the success of which could be recognised. They knew perfectly well what it was: the French pox.

Roger French is director, Wellcome Unit for the History of Medicine, University of Cambridge.