72 



SCIENCE. 



[Vol. XIV. No. 339 



infectious germs of these diseases are discharged from the body in 

 a condition incapable of producing immediate infection. In an 

 address delivered on another occasion, I have endeavored to present 

 the considerations which reconcile the comparative infrequency of 

 direct contagion for these diseases with the belief in the elimination 

 of the causative germs in an active state from the body, and have 

 there pointed out several well-known factors which determine the 

 frequency of conveyance of an infectious disease by contagion. 

 There are reasons, some of them very obvious, why diseases in 

 which the infectious substances are operative only when received 

 into the digestive tract, and are discharged usually only with the 

 fasces, are less likely to be transmitted by immediate contagion 

 than those diseases in which the virus is thrown off from the skin 

 on epidermal scales. 



But the field of operation of direct contagion for those so-called 

 miasmatic contagious diseases is at most a restricted one,' and the 

 chief sources of infection are outside of the body from which pri- 

 marily the infectious germs may have been derived. It is to these 

 external sources of infection, which are of such importance in pub- 

 lic hygiene, that I wish especially to direct attention. 



A full comprehension of the sources of infection is, of course, to 

 be obtained only by a detailed study of the etiology of the indi- 

 vidual infectious diseases ; but this is, of course, impossible within 

 the limits of an address. It may, however, be useful to present 

 some of the facts which have a general bearing upon the subject. 

 Let us consider, then, from the point of view of modern bacterio- 

 logical studies, what role in harboring or transporting infectious 

 agents may be played by those substances or media with which 

 we necessarily come into intimate contact, such as the air, the 

 ground, the water, and our food. 



It is universally admitted that many infectious agents may be 

 transported by the air, but the extent of danger from this source 

 has often been exaggerated. It is a popular error to suppose that 

 most of the minute particles of dust in the air either are or contain 

 living organisms. The methods for determining the number and 

 kind of bacteria and fungi in the air are now fairly satisfactory, 

 although by no means perfect. These have shown that while the 

 number of living bacteria and fungi in the atmosphere in and 

 around human habitations cannot be considered small, still it is 

 greatly inferior to that in the ground or in most waters. Unlike 

 fungus spores, bacteria do not seem to occur to any extent in the 

 air as single detached particles, which would then necessarily be 

 extremely minute, but rather in clumps or attached to particles of 

 dust of relatively large size. As a result, in a perfectly quiet at- 

 mosphere these comparatively heavy particles which contain bac- 

 teria rapidly settle to the ground or upon underlying objects, and 

 are easily filtered out bypassing the air through porous substances, 

 such as cotton-wool or sand. Rain washes down a large number 

 of the bacteria from .the air. That the air bacteria are derived 

 from the ground, or objects upon it, is shown by their total 

 absence, as a rule, from sea-air at a distance from land, this 

 distance naturally varying with the direction and strength of the 

 wind. 



A fact of capital importance in understanding the relations of 

 bacteria to the air, and one of great significance for preventive 

 medicine, is the impossibility of currents of air detaching bacteria 

 from moist surfaces. Substances containing pathogenic bacteria, 

 as, for instance, sputum containing tubercle bacilli, or excreta hold- 

 ing typhoid bacilli, cannot, therefore, infect the air unless these 

 substances first become dry and converted into a fine powder. We 

 are able to understand why the expired breath is free from bacteria 

 and cannot convey infection, except as little particles may be me- 

 chanically detached by acts of coughing, sneezing, or hawking. 

 Those bacteria the vitality of which is rapidly destroyed by com- 

 plete desiccation, such as those of Asiatic cholera, evidently are not 

 likely to be transported as infectious agents by the air, if we except 

 such occasional occurrences as their conveyance for a short dis- 

 tance in spray. 



The only pathogenic bacteria which hitherto have been found in 

 the air are the pus-organisms, including the streptococcus found 

 by Prudden in a series of cases of diphtheria and tubercle bacilli ; 

 but no far-reaching conclusions can be drawn from the failure to 

 find other infectious organisms, when we consider the imperfection 



of our methods, and the small number of observations directed to 

 this point. The evidence in other ways is conclusive that many 

 infectious agents — and here the malarial germ should be promi- 

 nently mentioned — can be, and often are, conveyed by the air. 

 While we are inclined to restrict within narrower limits than has 

 been customary the danger of infection through the air, we must 

 recognize that this still remains an important source of infection 

 for many diseases. All those, however, who have worked practi- 

 cally with. the cultivation of micro-organisms, have come to regard 

 contact with infected substances as more dangerous than exposure 

 to the air; and the same lesson may be learned from the methods 

 which modern surgeons have found best adapted to prevent the 

 infection of wounds with the cosmopolitan bacteria which cause 

 suppuration. 



We are not, of course, to suppose that infectious germs floating 

 in the form of dust in the atmosphere are dangerous only from the 

 possibility of our drawing them in with the breath. Such germs 

 may be deposited on substances with which we readily come into 

 contact, or they may fall on articles of food where they may find 

 conditions suitable for their reproduction, which cannot occur 

 when they are suspended in the air, in consequence of the lack of 

 moisture. 



From the facts which have been mentioned concerning the rela- 

 tions of bacteria to the air, what points of view present themselves 

 to guide us in preventing infection through this channel ? Surely 

 something more than that this purpose is accomplished simply by 

 abolishing foul odors. 



Certain indications are so plain as to need only to be mentioned 

 in this connection, such as the disinfection and removal, as far as 

 possible, of all infected substances, — an indication which applies 

 equally to all channels of infection, and which is much easier to 

 mention than it is to describe how it shall be realized. But there 

 are two indications which apply especially to the prevention .of 

 the transportation of disease-germs by the air. One is the neces- 

 sity of guarding, so far as practicable, against the desiccation, 

 when exposed to the air, of substances which contain infectious 

 germs not destroyed by drying ; and another is free ventilation. 



For no disease is the importance of the first of these indications 

 so evident and so well established as for tuberculosis, the most 

 devastating of all infectious diseases. Against this disease, formi- 

 dable as it may seem to cope with it, the courageous crusade of 

 preventive medicine has begun, and is destined to continue. 



It is now generally recognized that the principal, although not 

 the sole, sources of tuberculous infection are the sputum of indi- 

 viduals affected with pulmonary tuberculosis, and the milk of tu- 

 berculous cows. Cornet, who has made a laborious and most in- 

 structive experimental study ofuhe modes and dangers of infection 

 from tuberculous sputum, has also elaborated the practical meas- 

 ures which should be adopted to diminish or annihilate those dan- 

 gers. These measures have been so recently and so widely pub- 

 lished in medical journals, and so clearly presented before a section 

 of this association, that I mention them only to call the attention of 

 practitioners of medicine to their importance, and to emphasize the 

 fact that they are based chiefiy upon the principle that infectious 

 substances of such nature as tuberculous sputum should not be 

 allowed to become dry and converted into dust when exposed to 

 the air. 



By means of free ventilation, disease-producing micro-organisms 

 which may be present in the air of rooms are carried away, and 

 distributed so far apart that the chance of infection from this 

 source is removed, or reduced to a minimum. It is a well-estab- 

 lished clinical observation that the distance through which the 

 specific microbes of such diseases as small-pox or scarlatina are 

 Ukely to be carried from the patient by the air in such concentra- 

 tion as to cause infection, is small, usually not more than a few 

 feet, but increases by crowding of patients and absence of free ven- 

 tilation. The well-known experiences in the prophylaxis and 

 treatment of typhus-fever are a forcible illustration of the value of 

 free ventilation. 



It is, of course, not to be understood that by ventilation we ac- 

 complish the disinfection of a house or apartment. Ventilation is 

 only an adjunct of such disinfection, which, as already mentioned, 

 is of first importance. Time will not permit, nor is it in the plan 



