Darwin is discussing diseases transmitted from Europeans to the natives. Note that he has no suspicion even of the “germ theory of disease”.(except for measles)
“The Rev. J. Williams, in his interesting work, 2 says, that the first intercourse between natives and Europeans, “is invariably attended with the introduction of fever, dysentery, or some other disease, which carries off numbers of the people.” Again he affirms, “It is certainly a fact, which cannot be controverted, that most of the diseases which have raged in the islands during my residence there, have been introduced by ships; 3 and what renders this fact remarkable is, that there might be no appearance of disease among the crew of the ship which conveyed this destructive importation.” This statement is not quite so extraordinary as it at first appears; for several cases are on record of the most malignant fevers having broken out, although the parties themselves, who were the cause, were not affected. In the early part of the reign of George III., a prisoner who had been confined in a dungeon, was taken in a coach with four constables before a magistrate; and although the man himself was not ill, the four constables died from a short putrid fever; but the contagion extended to no others. From these facts it would almost appear as if the effluvium of one set of men shut up for some time together was poisonous when inhaled by others; and possibly more so, if the men be of different races. Mysterious as this circumstance appears to be, it is not more surprising than that the body of one’s fellow-creature, directly after death, and before putrefaction has commenced, should often be of so deleterious a quality, that the mere puncture from an instrument used in its dissection, should prove fatal. " “