here is one possible explanation:
The principle of 'counter-irritation'
The remedy wouldn't have seemed so strange to doctors a hundred years ago, who often prescribed liniments and poultices containing mild irritants such as mustard, garlic, or camphor to the chest and to the soles of the feet to relieve symptoms of colds and whooping cough. Like Vicks VapoRub -- the active ingredients of which include camphor, eucalyptus, and menthol -- these preparations would have had the effect of stimulating blood flow to the skin. Catalogued under the heading of "counter-irritants" in early twentieth-century medical texts, such treatments were based on the principle that "internal morbid processes may at times be relieved by creating external irritations" (Horatio Charles Wood in Therapeutics: Its Principles and Practice, 1908).
To be sure, there was vigorous debate over how counter-irritants actually worked. "One commonly offered explanation," wrote pharmacologist Horatio Wood at the time, "is that there is only a certain amount of blood in the body, and that if the blood be drawn to one part there must be less in another part. Surely, however, the amount of blood drawn to the skin by a mustard plaster is too small sensibly to affect the general mass in the body. It is more probable that the phenomena of counter-irritation are the result of reflex disturbances of the vaso-motor nerves which influence the size of the blood vessels, or of the trophic nerves which directly affect nutrition."
