9th. It is transportable in fomites through great distances, either on sea or land, and as often as its toxic effects are manifested after these portations they are so uniform as to be promptly recognizable.
A great number of different materials in common use may act as fomites, such as loose wool, cotton, or hair, or textile fabrics of various descriptions.
The following facts, which illustrate how yellow-fever infection may be conveyed in the most unsuspecting and innocent manner, are well authenticated. There can be no ground for accusation of error except in the hypothesis that the infection was encountered simultaneously in some unexplained manner. The facts are furnished by Dr. Shannon of Ocean Springs, Mississippi: "On the 14th of October, 1883, Maj. J. B. B. died of yellow fever in Ocean Springs, Miss. I moved the family at once to the healthy locality where you saw Miss B., not allowing them to take any article from the room where the husband and father had died. The children applied to me for a lock of their father's hair, which I refused, but the oldest daughter, now dead, prevailed upon the nurse to give it her. She placed it in an old envelope that had been torn open at the end and carefully folded the torn end down, thus practically sealing it, and laid it away among other old letters. On Sunday, the 4th of November, at 12.30 P.M., she brought this envelope out upon the open gallery, and opened it for the first time to examine the lock of hair and show it to her aunt, Miss S., who was visiting her, and upon inhaling the concentrated poison confined in the envelope and emanating from the hair, exclaimed, 'Oh, what a peculiar smell!' She then handed the envelope to her aunt, Miss S., who, unconscious of danger, also inhaled the 'messenger of death' with a similar exclamation, when Mrs. B., who was standing near, reached out her hand for the envelope, but was prevented from getting it by the entreaties of a fretful child to be taken up in her arms. This gave time for sufficient reflection, and she admonished the young ladies of the possible danger. The envelope was then carefully folded, and with its fatal contents replaced in the drawer where it had been since the 14th of October. This drawer had been almost daily opened. On the following Saturday night, Nov. 10th, at 9 P.M., Miss S. was taken sick with a chill, and Miss B. at about 2 A.M., some five hours later, the period of incubation being less than seven days in both cases. No other person handled the fatal envelope or in any way came in contact with it, and there is, after the most careful inquiry, no suspicion of any other source of infection in these two cases. Miss S. died on Oct. 14th, Miss B. on Oct. 16th."
10th. These qualities of yellow-fever infection, and especially its faculty of reproduction (which only organisms possess), furnish almost conclusive evidence that yellow fever is a germ disease produced by a specific contagium vivum.
Many facts are patent which sustain the generally accepted opinion that yellow-fever poison gains admission to the system through the medium of atmospheric air. On the other hand, I know of no observations which prove that the disease is ever communicated by food or drinks, or through any other vehicle than atmospheric air.
In respect to atmospheric infection by yellow fever, localizations of aërial impregnation are often observable, not common in other air-infecting diseases. A certain district of a large and populous city may become the seat of a sweeping and fatal epidemic, and yet no case occur outside of this area of prevalence. It is customary to speak of these points of epidemic prevalence as infected localities. If unprotected persons visit such infected places, even for a short period of time, they are liable to attacks of yellow fever, although they may take neither food nor drink within the limits of infection and bring no fomites away with them. Under these circumstances atmospheric impregnation is conclusive.
But it is difficult to determine how this infection of a locality has been produced in the first place, and how, in the second place, it is maintained sometimes for periods of from one to three months, with so little apparent diminution or change in the liability to communicate yellow fever to unprotected visitors within the limits of infection.
It seems highly probable that yellow-fever poison, after its exit from the human body, attaches itself to various solid surfaces in proximity to the sick, where, under suitable climatic conditions, it undergoes more or less speedy processes of maturation in toxic qualities. The poison thus matured is capable of being preserved with but little change for the periods indicated above, and is communicable through the atmosphere for short distances. It is also capable, by virtue of some unexplained process or quality, of spontaneously extending its area of infection. But this is at all times slow, and is readily interrupted by streams of water, high walls, or even by much-travelled thoroughfares.
There are no instances in which the water-supply of cities has been shown to have distributed yellow fever.
The periods of time which may intervene between exposure to yellow-fever poison and attacks of the disease are extremely variable. The shortest period of incubation which has come under my observation was about twenty hours. In three cases in which I was able to fix the hours of first exposure with precision attacks followed in 72 hours, 83 hours, and 101 hours, respectively. Of 55 unacclimated physicians who exposed themselves at Memphis during the epidemic of 1878, 54 suffered attacks of yellow fever. In these cases the periods of incubation varied from one to twenty-five days, the average duration being ten days. These physicians all remained steadfastly at their posts of duty; consequently, the attack which occurred on the twenty-fifth day was postponed for that length of time during constant exposure in a locality most intensely infected.