Several brief histories of allergies turn up in any search engine (a few obviously heavily plagiarized, unfortunately), but the most extensive and informative seems to be a corporate newsletter's three-part summary of the book Ancestors of Allergy (1994), edited by Estelle Simons: Part 1 (Ancient World), Part 2 (Middle Ages and Renaissance), Part 3 (Modern). On the skin condition side, the available authority seems to be the Historical Atlas of Dermatology and Dermatologists (2002) by John Crissey, Lawrence Parish and Karl Holubar.
It turns out that the word "allergy" was coined in 1906, and "analphylaxis" in 1902. How long those words took to enter the common vocabulary and understanding, I don't know, but I would guess that they remained medical/academic specialty concepts for some years.
For most of Nick's existence, right into the nineteenth century, the sophisticated would have regarded any allergic symptoms (rash, sneezing, etc.) as evidence of an imbalance of the "humors" (black bile, yellow bile, phlegm, blood), while the ordinary person would likely have regarded them as evidence of a curse or demonic interference, or punishment for a sin and sign of God's disfavor. (I speculate that Nick himself continues to view his sunlight intolerance as precisely that, curse or punishment, down to the present day.)
In the middle ages, Persia and Arabia developed the idea of "rose fever," what we today call seasonal respiratory allergies. In Europe, however, this idea did not take hold until the Renaissance, when it was called "rose catarrh" (later "rose cold," later "hay fever"). For some time, it was thought to be a disease only of the upper classes (who perhaps had leisure and means to have it diagnosed and treated, unlike poorer sufferers). Early works mentioning treatments for the wide array of human skin conditions include The Canon of Medicine (1025) by Avicenna, which prescribed zinc oxide for what we today call skin cancer, and De Morbis Cutaneis (1572) by Geronimo Mercuriali, which is transcriptions of detailed lectures to medical students.
All ancient civilizations observed the symptoms of both asthma and allergies, but while most identified and named asthma, none seem to have named allergies or considered them a class of related ailments. Some historical incidents that feature what we today recognize as allergies, but which were inexplicable at the time, include Pharaoh Menes dying after being stung by a wasp; Emperor Nero inheriting because his brother Britannicus's eyes swelled shut around horses; and King Richard III using his chronic reaction to strawberries as an excuse to kill Lord William Hastings (claiming that Hastings had cursed him).
At this time, I conclude that allergies are not a highly useful cover for Nick's condition before the latter half of the twentieth century, but that the humoral understanding of skin conditions may yield some plausible excuses. More research!
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