Philada Jany. 20—1806.
I beg leave to inform you that the College of Physicians of Philada. have sent by the mail which carries this a Pamphlet containing “Additional Facts & Observations relative to the Pestilential Fever,” & that they request you to accept it as a mark of their respect—I believe it contains indisputable proofs of a Contagious quality in the yellow fever, in the instances there specified, & if this should be established in a single point, you well know the importance of its application to the general principle—That the Disease has not exhibited its Contagious nature in a vast number of instances is very Certain, but these negative facts no more invalidate the inferences drawn from the positive, than the inability of Electricity to pervade glass would disprove its power of penetrating Iron or splitting timber. After Comparing the apparently Contradictory facts, does it not seem probable that there exists some power which Counteracts its Contagious quality, in many places, but which is often absent, & these the Contagion acts like that of the Small pox or Measles or other diseases of acknowledged Contagion. It seems really to have acted so in Germantown, in 1798, as you will see in the Pamphlet, I mention this Case with more Confidence as I am well acquainted with the family & Neighbourhood, & believe that Monticello is not more healthy than that part of the Village—This has been the character of the place for 100 Years indeed from its first settlement—Yet after the appearance of the Yellow Fever in a Child, who had recently left a Sickly part of Philada, eight or nine persons were successively attacked with the same disease—All their neighbours, out of the sphere of these Patients Continuing healthy—Have we any better reasons for Considering the Small Pox or Measles as Contagious. I must however not intrude farther on your time, but I cannot close without saying that I have ascertained the head you sent us from Dr Brown to be that of the Pecari or Hog of America, & also that we have received the Scileton of an Elephant from the Asiatic Society, which is to be placed along side of the Mammoth. I shall proceed as fast as my imperious professional Calls will permit to Compare them & Ascertain the difference—
With sincerity & affection Your obliged friend
C Wistar Junr.
DLC: Papers of Thomas Jefferson.