Diseases can go unrecognised for decades or centuries – until some clever clinician sees a pattern that no one else has seen. A few years ago, a previously carnivorous relative of mine became ill – swelling and gut symptoms – after eating meat. Fortunately a knowledgable doctor recognised this as the recently described tick-induced allergy to mammalian meat. Ceasing all red meat stopped the illness episodes. Tick-induced mammalian meat allergy was first described by a Sydney doctor in 2007. The Australian paralysis tick is responsible for inducing this allergy to a carbohydrate present in all mammals, except humans and apes. This meat allergy has now been recognised in many countries covering all continents (except Antarctica which currently lacks ticks). The condition was not new, but its recognition was.
The number of diseases we now recognise has grown to at least 20,000 conditions, with almost half being rare genetic disorders. We don’t know how many more will eventually be recognised but currently around 4 new diseases are being described per week. Occasionally this is a new diseases – such as HIV or COVID-19 – but most a likely diseases that have been around for a long time but previously unrecognised. The recognition of new diseases tends to come in waves associated with changes in disease paradigm. For example, the germ theory of disease, aided by the microscope, allowed the medical community to elucidate the large range of infectious diseases. More recently, the ability to cheaply sequence DNA has greatly expanded the ability to recognise rare genetic disorders. But sometimes recognition is just careful clinical observation, as illustrated by the mammalian meat allergy or Oliver Sack’s descriptions of unusual neurological cases set out in his books such as The Man Who Mistook His Wife for a Hat.
Both the growth and the total number of diseases have implications for medical practice and education. We don’t know the number of still unrecognised diseases, but the current growth rate suggests there must be many more unrecognised conditions. This should give clinicians and the medical community a sense of humility in the face of puzzling patients. These could be thought of as either our collective ignorance or the “unknown unknowns” of the Johari window (often attributed to Donald Rumsfeld). But puzzling patients can also arise from individual clinician ignorance – conditions that are known in the medical literature but unknown by the individual clinician. That is hardly surprising: no clinician can know all the 20,000 current conditions plus the four new conditions per week. A modicum of humility is needed in the face of the twin mountains of our extensive knowledge and ignorance.
Medical educators should impart to new students the limitations of our knowledge and the unknowability of all of the diseases that we currently recognize – learning one new disease per day would take 55 years to learn them all (assuming no forgetting!) Fortunately, clinical presentations follow a very skewed distribution. In an analysis of the reasons for clinical encounter for GPs we found that around 35 conditions could explain 50% of consultations. However the remaining 50% had a very long tail comprising hundreds of conditions (and of course potentially all 20,000 – most of which a GP will never see). So a GP needs to know those top 30+ conditions very well, but also have ways to manage the 19,000+ conditions they may see, which every week is likely to include a condition they have not seen before.
Healthy Evidence 